Fix Fetcher retriable error handling (#2554)

Author: dpkp

kafka/consumer/fetcher.py

@@ -934,9 +934,9 @@ def _parse_fetched_data(self, completed_fetch):
             elif error_type is Errors.TopicAuthorizationFailedError:
                 log.warning("Not authorized to read from topic %s.", tp.topic)
                 raise Errors.TopicAuthorizationFailedError(set([tp.topic]))
-            elif error_type.is_retriable:
+            elif getattr(error_type, 'retriable', False):
                 log.debug("Retriable error fetching partition %s: %s", tp, error_type())
-                if error_type.invalid_metadata:
+                if getattr(error_type, 'invalid_metadata', False):
                     self._client.cluster.request_update()
             else:
                 raise error_type('Unexpected error while fetching data')