Index: src/wp-admin/includes/ajax-actions.php
===================================================================
--- src/wp-admin/includes/ajax-actions.php	(revision 38188)
+++ src/wp-admin/includes/ajax-actions.php	(working copy)
@@ -3345,16 +3345,25 @@
 		wp_send_json_error( $status );
 	}
 
-	$upgrader = new Theme_Upgrader( new Automatic_Upgrader_Skin() );
+	$skin     = new WP_Ajax_Upgrader_Skin();
+	$upgrader = new Theme_Upgrader( $skin );
 	$result   = $upgrader->install( $api->download_link );
 
 	if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {
-		$status['debug'] = $upgrader->skin->get_upgrade_messages();
+		$status['debug'] = $skin->get_upgrade_messages();
 	}
 
 	if ( is_wp_error( $result ) ) {
+		$status['errorCode']    = $result->get_error_code();
 		$status['errorMessage'] = $result->get_error_message();
 		wp_send_json_error( $status );
+	} elseif ( is_wp_error( $skin->result ) ) {
+		$status['errorCode']    = $skin->result->get_error_code();
+		$status['errorMessage'] = $skin->result->get_error_message();
+		wp_send_json_error( $status );
+	} elseif ( $skin->get_errors()->get_error_code() ) {
+		$status['errorMessage'] = $skin->get_error_messages();
+		wp_send_json_error( $status );
 	} elseif ( is_null( $result ) ) {
 		global $wp_filesystem;
 
@@ -3363,6 +3372,7 @@
 
 		// Pass through the error from WP_Filesystem if one was raised.
 		if ( $wp_filesystem instanceof WP_Filesystem_Base && is_wp_error( $wp_filesystem->errors ) && $wp_filesystem->errors->get_error_code() ) {
+			$status['errorCode']    = $wp_filesystem->errors->get_error_code();
 			$status['errorMessage'] = esc_html( $wp_filesystem->errors->get_error_message() );
 		}
 
@@ -3437,14 +3447,26 @@
 		wp_update_themes();
 	}
 
-	$upgrader = new Theme_Upgrader( new Automatic_Upgrader_Skin() );
+	$skin     = new WP_Ajax_Upgrader_Skin();
+	$upgrader = new Theme_Upgrader( $skin );
 	$result   = $upgrader->bulk_upgrade( array( $stylesheet ) );
 
 	if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {
-		$status['debug'] = $upgrader->skin->get_upgrade_messages();
+		$status['debug'] = $skin->get_upgrade_messages();
 	}
 
-	if ( is_array( $result ) && ! empty( $result[ $stylesheet ] ) ) {
+	if ( is_wp_error( $result ) ) {
+		$status['errorCode']    = $result->get_error_code();
+		$status['errorMessage'] = $result->get_error_message();
+		wp_send_json_error( $status );
+	} elseif ( is_wp_error( $skin->result ) ) {
+		$status['errorCode']    = $skin->result->get_error_code();
+		$status['errorMessage'] = $skin->result->get_error_message();
+		wp_send_json_error( $status );
+	} elseif ( $skin->get_errors()->get_error_code() ) {
+		$status['errorMessage'] = $skin->get_error_messages();
+		wp_send_json_error( $status );
+	} elseif ( is_array( $result ) && ! empty( $result[ $stylesheet ] ) ) {
 
 		// Theme is already at the latest version.
 		if ( true === $result[ $stylesheet ] ) {
@@ -3458,10 +3480,6 @@
 		}
 
 		wp_send_json_success( $status );
-	} elseif ( is_wp_error( $upgrader->skin->result ) ) {
-		$status['errorCode']    = $upgrader->skin->result->get_error_code();
-		$status['errorMessage'] = $upgrader->skin->result->get_error_message();
-		wp_send_json_error( $status );
 	} elseif ( false === $result ) {
 		global $wp_filesystem;
 
@@ -3470,6 +3488,7 @@
 
 		// Pass through the error from WP_Filesystem if one was raised.
 		if ( $wp_filesystem instanceof WP_Filesystem_Base && is_wp_error( $wp_filesystem->errors ) && $wp_filesystem->errors->get_error_code() ) {
+			$status['errorCode']    = $wp_filesystem->errors->get_error_code();
 			$status['errorMessage'] = esc_html( $wp_filesystem->errors->get_error_message() );
 		}
 
@@ -3594,16 +3613,25 @@
 
 	$status['pluginName'] = $api->name;
 
-	$upgrader = new Plugin_Upgrader( new Automatic_Upgrader_Skin() );
+	$skin     = new WP_Ajax_Upgrader_Skin();
+	$upgrader = new Plugin_Upgrader( $skin );
 	$result   = $upgrader->install( $api->download_link );
 
 	if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {
-		$status['debug'] = $upgrader->skin->get_upgrade_messages();
+		$status['debug'] = $skin->get_upgrade_messages();
 	}
 
 	if ( is_wp_error( $result ) ) {
+		$status['errorCode']    = $result->get_error_code();
 		$status['errorMessage'] = $result->get_error_message();
 		wp_send_json_error( $status );
+	} elseif ( is_wp_error( $skin->result ) ) {
+		$status['errorCode']    = $skin->result->get_error_code();
+		$status['errorMessage'] = $skin->result->get_error_message();
+		wp_send_json_error( $status );
+	} elseif ( $skin->get_errors()->get_error_code() ) {
+		$status['errorMessage'] = $skin->get_error_messages();
+		wp_send_json_error( $status );
 	} elseif ( is_null( $result ) ) {
 		global $wp_filesystem;
 
@@ -3612,6 +3640,7 @@
 
 		// Pass through the error from WP_Filesystem if one was raised.
 		if ( $wp_filesystem instanceof WP_Filesystem_Base && is_wp_error( $wp_filesystem->errors ) && $wp_filesystem->errors->get_error_code() ) {
+			$status['errorCode']    = $wp_filesystem->errors->get_error_code();
 			$status['errorMessage'] = esc_html( $wp_filesystem->errors->get_error_message() );
 		}
 
@@ -3680,19 +3709,26 @@
 
 	wp_update_plugins();
 
-	$skin     = new Automatic_Upgrader_Skin();
+	$skin     = new WP_Ajax_Upgrader_Skin();
 	$upgrader = new Plugin_Upgrader( $skin );
 	$result   = $upgrader->bulk_upgrade( array( $plugin ) );
 
 	if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {
-		$status['debug'] = $upgrader->skin->get_upgrade_messages();
+		$status['debug'] = $skin->get_upgrade_messages();
 	}
 
-	if ( is_array( $result ) && empty( $result[ $plugin ] ) && is_wp_error( $skin->result ) ) {
-		$result = $skin->result;
-	}
-
-	if ( is_array( $result ) && ! empty( $result[ $plugin ] ) ) {
+	if ( is_wp_error( $result ) ) {
+		$status['errorCode']    = $result->get_error_code();
+		$status['errorMessage'] = $result->get_error_message();
+		wp_send_json_error( $status );
+	} elseif ( is_wp_error( $skin->result ) ) {
+		$status['errorCode']    = $skin->result->get_error_code();
+		$status['errorMessage'] = $skin->result->get_error_message();
+		wp_send_json_error( $status );
+	} elseif ( $skin->get_errors()->get_error_code() ) {
+		$status['errorMessage'] = $skin->get_error_messages();
+		wp_send_json_error( $status );
+	} elseif ( is_array( $result ) && ! empty( $result[ $plugin ] ) ) {
 		$plugin_update_data = current( $result );
 
 		/*
@@ -3716,9 +3752,6 @@
 			$status['newVersion'] = sprintf( __( 'Version %s' ), $plugin_data['Version'] );
 		}
 		wp_send_json_success( $status );
-	} elseif ( is_wp_error( $result ) ) {
-		$status['errorMessage'] = $result->get_error_message();
-		wp_send_json_error( $status );
 	} elseif ( false === $result ) {
 		global $wp_filesystem;
 
Index: src/wp-admin/includes/class-wp-upgrader-skins.php
===================================================================
--- src/wp-admin/includes/class-wp-upgrader-skins.php	(revision 38188)
+++ src/wp-admin/includes/class-wp-upgrader-skins.php	(working copy)
@@ -36,3 +36,6 @@
 
 /** Automatic_Upgrader_Skin class */
 require_once ABSPATH . 'wp-admin/includes/class-automatic-upgrader-skin.php';
+
+/** WP_Ajax_Upgrader_Skin class */
+require_once ABSPATH . 'wp-admin/includes/class-wp-ajax-upgrader-skin.php';
Index: src/wp-admin/includes/class-wp-upgrader.php
===================================================================
--- src/wp-admin/includes/class-wp-upgrader.php	(revision 38188)
+++ src/wp-admin/includes/class-wp-upgrader.php	(working copy)
@@ -39,6 +39,9 @@
 /** Automatic_Upgrader_Skin class */
 require_once ABSPATH . 'wp-admin/includes/class-automatic-upgrader-skin.php';
 
+/** WP_Ajax_Upgrader_Skin class */
+require_once ABSPATH . 'wp-admin/includes/class-wp-ajax-upgrader-skin.php';
+
 /**
  * Core class used for upgrading/installing a local set of files via
  * the Filesystem Abstraction classes from a Zip file.
Index: src/wp-admin/js/updates.js
===================================================================
--- src/wp-admin/js/updates.js	(revision 38188)
+++ src/wp-admin/js/updates.js	(working copy)
@@ -244,7 +244,7 @@
 	 * @param {string=} response.errorCode Optional. Error code for an error that occurred.
 	 */
 	wp.updates.ajaxAlways = function( response ) {
-		if ( ! response.errorCode && 'unable_to_connect_to_filesystem' !== response.errorCode ) {
+		if ( ! response.errorCode || 'unable_to_connect_to_filesystem' !== response.errorCode ) {
 			wp.updates.ajaxLocked = false;
 			wp.updates.queueChecker();
 		}
@@ -1524,7 +1524,7 @@
 	 * @returns {boolean} Whether there is an error that needs to be handled or not.
 	 */
 	wp.updates.maybeHandleCredentialError = function( response, action ) {
-		if ( response.errorCode && 'unable_to_connect_to_filesystem' === response.errorCode ) {
+		if ( wp.updates.shouldRequestFilesystemCredentials && response.errorCode && 'unable_to_connect_to_filesystem' === response.errorCode ) {
 			wp.updates.credentialError( response, action );
 			return true;
 		}