Return 500 on exception during asset combining (#4142)

Credit to @alekseyp

modules/system/classes/SystemController.php

@@ -4,6 +4,7 @@
 use ApplicationException;
 use Illuminate\Routing\Controller as ControllerBase;
 use Exception;
+use Response;
 
 /**
  * The is the master controller for system related routing.
@@ -36,7 +37,7 @@ public function combine($name)
 
         }
         catch (Exception $ex) {
-            return '/* '.e($ex->getMessage()).' */';
+            return Response::make('/* '.e($ex->getMessage()).' */', 500);
         }
     }
 }