Fix messages API to not NPE

show_if does not work on a method route

endpoints/api/globalmessages.py

@@ -71,9 +71,11 @@ class GlobalUserMessages(ApiResource):
   @nickname('createGlobalMessage')
   @validate_json_request('CreateMessage')
   @require_scope(scopes.SUPERUSER)
-  @show_if(features.SUPER_USERS)
   def post(self):
     """ Create a message """
+    if not features.SUPER_USERS:
+      abort(404)
+
     if SuperUserPermission().can():
       model.message.create([request.get_json()['message']])
       return make_response('', 201)