Index: src/wp-admin/includes/ajax-actions.php
===================================================================
--- src/wp-admin/includes/ajax-actions.php	(revision 38175)
+++ src/wp-admin/includes/ajax-actions.php	(working copy)
@@ -3353,6 +3353,7 @@
 	}
 
 	if ( is_wp_error( $result ) ) {
+		$status['errorCode']    = $result->get_error_code();
 		$status['errorMessage'] = $result->get_error_message();
 		wp_send_json_error( $status );
 	} elseif ( is_null( $result ) ) {
@@ -3602,6 +3603,7 @@
 	}
 
 	if ( is_wp_error( $result ) ) {
+		$status['errorCode']    = $result->get_error_code();
 		$status['errorMessage'] = $result->get_error_message();
 		wp_send_json_error( $status );
 	} elseif ( is_null( $result ) ) {
Index: src/wp-admin/includes/class-wp-upgrader.php
===================================================================
--- src/wp-admin/includes/class-wp-upgrader.php	(revision 38175)
+++ src/wp-admin/includes/class-wp-upgrader.php	(working copy)
@@ -549,7 +549,8 @@
 			$_files = $wp_filesystem->dirlist($remote_destination);
 			if ( ! empty($_files) ) {
 				$wp_filesystem->delete($remote_source, true); //Clear out the source files.
-				return new WP_Error('folder_exists', $this->strings['folder_exists'], $remote_destination );
+				$this->result = new WP_Error('folder_exists', $this->strings['folder_exists'], $remote_destination );
+				return $this->result;
 			}
 		}
 
Index: src/wp-admin/js/updates.js
===================================================================
--- src/wp-admin/js/updates.js	(revision 38175)
+++ src/wp-admin/js/updates.js	(working copy)
@@ -526,7 +526,7 @@
 		$card.removeClass( 'plugin-card-install-failed' ).find( '.notice.notice-error' ).remove();
 
 		$document.trigger( 'wp-plugin-installing', args );
-		
+
 		return wp.updates.ajax( 'install-plugin', args );
 	};
 
@@ -718,7 +718,7 @@
 		wp.a11y.speak( wp.updates.l10n.deleting, 'polite' );
 
 		$document.trigger( 'wp-plugin-deleting', args );
-		
+
 		return wp.updates.ajax( 'delete-plugin', args );
 	};
 
@@ -1043,7 +1043,7 @@
 		$( '.install-theme-info, [data-slug="' + args.slug + '"]' ).removeClass( 'theme-install-failed' ).find( '.notice.notice-error' ).remove();
 
 		$document.trigger( 'wp-theme-installing', args );
-		
+
 		return wp.updates.ajax( 'install-theme', args );
 	};
 
@@ -1174,7 +1174,7 @@
 		$( '.theme-info .update-message' ).remove();
 
 		$document.trigger( 'wp-theme-deleting', args );
-		
+
 		return wp.updates.ajax( 'delete-theme', args );
 	};
 
@@ -1525,7 +1525,7 @@
 	 * @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;
 		}
@@ -1982,7 +1982,7 @@
 			$bulkActionForm.find( '.manage-column [type="checkbox"]' ).prop( 'checked', false );
 
 			$document.trigger( 'wp-' + type + '-bulk-' + bulkAction, itemsSelected );
-			
+
 			// Find all the checkboxes which have been checked.
 			itemsSelected.each( function( index, element ) {
 				var $checkbox  = $( element ),