Add an exception message

jingming · jingming · commit ed160fda8af2 · 2017-03-07T10:31:46.000-08:00
diff --git a/twilio/twiml/__init__.py b/twilio/twiml/__init__.py
@@ -61,7 +61,7 @@ def append(self, verb):
         :return:
         """
         if not isinstance(verb, TwiML):
-            raise TwiMLException()
+            raise TwiMLException('Only appending of TwiML is allowed')
 
         self.verbs.append(verb)
         return self
