code: def _get_error(self, stanza):
          
          if stanza:
              logger.debug(u"Roster request failed: {0}".format(
                                                  stanza.error.condition_name))
          else:
              logger.debug(u"Roster request failed: timeout")
          self._event_queue.put(RosterNotReceivedEvent(self, stanza))
docs: Handle failure of the roster request.