Improve exception message for missing selector

src/Drupal/ContentTypeRegistry/Widgets/MediaBrowserWidget.php

@@ -45,7 +45,7 @@ public function fill($I, $value)
 
         if (!preg_match('/^#edit\-([\w\-]+)\-upload$/', $selector, $matches)) {
             throw new \InvalidArgumentException(
-                "Must specify the input field, beginning with edit- and ending in -upload"
+                "Must specify the input field, beginning with '#edit-' and ending in '-upload'."
             );
         }