better error message when we can't retrieve resource locations.

Author: tedchamb

vsts/vsts/vss_client.py

@@ -158,6 +158,8 @@ def _get_resource_locations(self, all_host_types):
             headers['X-TFS-FedAuthRedirect'] = 'Suppress'
         response = self._send_request(request, headers=headers)
         wrapper = self._base_deserialize('VssJsonCollectionWrapper', response)
+        if wrapper is None:
+            raise VstsClientRequestError("Failed to retrieve resource locations from: {}".format(options_uri))
         collection = wrapper.value
         returned_locations = self._base_deserialize('[ApiResourceLocation]',
                                                     collection)