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)
@@ -3601,6 +3601,10 @@
 		$status['debug'] = $upgrader->skin->get_upgrade_messages();
 	}
 
+	if ( is_wp_error( $upgrader->skin->result ) ) {
+		$result = $upgrader->skin->result;
+	}
+
 	if ( is_wp_error( $result ) ) {
 		$status['errorMessage'] = $result->get_error_message();
 		wp_send_json_error( $status );