Update/Install error messages: do not escape from the template, escape the error message string before inserting it.

Props swissspidy, ocean90.
Fixes #37623 for trunk.

git-svn-id: https://develop.svn.wordpress.org/trunk@38240 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/js/updates.js

@@ -1608,7 +1608,7 @@
 		wp.updates.addAdminNotice( {
 			id:        'unknown_error',
 			className: 'notice-error is-dismissible',
-			message:   errorMessage
+			message:   _.escape( errorMessage )
 		} );
 
 		// Remove the lock, and clear the queue.