Index: src/wp-admin/js/updates.js
===================================================================
--- src/wp-admin/js/updates.js	(revision 38182)
+++ 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 );
 	};
 
@@ -1556,6 +1556,8 @@
 
 		if ( _.isString( response ) ) {
 			error = response;
+		} else if ( 'undefined' !== typeof response.readyState && 0 == response.readyState ) {
+			error = wp.updates.l10n.connectionError;
 		} else if ( _.isString( response.responseText ) && '' !== response.responseText ) {
 			error = response.responseText;
 		} else if ( _.isString( response.statusText ) ) {
@@ -1982,7 +1984,7 @@
 			$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 ),
Index: src/wp-includes/script-loader.php
===================================================================
--- src/wp-includes/script-loader.php	(revision 38182)
+++ src/wp-includes/script-loader.php	(working copy)
@@ -679,6 +679,7 @@
 				/* translators: %s: Importer name */
 				'activateImporterLabel'      => __( 'Run %s' ),
 				'unknownError'               => __( 'An unknown error occurred' ),
+				'connectionError'            => __( 'Connection lost or the server is busy. Please try again later.' ),
 				'pluginsFound'               => __( 'Number of plugins found: %d' ),
 				'noPluginsFound'             => __( 'No plugins found. Try a different search.' ),
 			),
Index: tests/qunit/fixtures/updates.js
===================================================================
--- tests/qunit/fixtures/updates.js	(revision 38182)
+++ tests/qunit/fixtures/updates.js	(working copy)
@@ -46,6 +46,7 @@
 		'activateThemeLabel':  'Activate %s',
 		'activateImporter': 'Run Importer',
 		'unknownError': 'An unknown error occurred',
+		'connectionError': 'Connection lost or the server is busy. Please try again later.',
 		'pluginsFound': 'Number of plugins found: %d',
 		'noPluginsFound': 'No plugins found. Try a different search.'
 	}