fix(taskqueue): ensure retryable tasks can not return null

taskqueue/gcloud/aio/taskqueue/utils.py

@@ -97,13 +97,14 @@ async def raise_for_status(resp):
 async def retry(coro, exceptions=None, retries=3):
     for attempt in range(retries):
         try:
-            return await coro
+            resp = await coro
+            if resp is not None:
+                return resp
         except Exception as e:  # pylint: disable=broad-except
             if exceptions is not None and e not in exceptions:
                 raise
             if attempt >= retries - 1:
                 raise
             log.warning('retrying due to %s', e)
 
-    raise aiohttp.client_exceptions.ClientResponseError(
-        None, None, code=429, message='hit retry limit ({})'.format(retries))
+    raise Exception('hit retry limit ({})'.format(retries))