CC-4248: Playlist Builder: 'Something went wrong' error message when dragging playlist into playlist

-included webstreams in the allowance type list in the error message

airtime_mvc/application/controllers/PlaylistController.php

@@ -155,7 +155,7 @@ class PlaylistController extends Zend_Controller_Action
 
     private function wrongTypeToPlaylist($obj)
     {
-        $this->view->error = "You can only add tracks and smart blocks to playlists.";
+        $this->view->error = "You can only add tracks, smart blocks, and webstreams to playlists.";
         $this->createFullResponse($obj);
     }