docs: def mark_failed(self, dispatch, error_log):
          """Marks a dispatch as failed.
  
          Sitemessage won't try to deliver already failed messages.
  
          Should be used within send().
  
          :param Dispatch dispatch: a Dispatch
          :param str error_log: str - error message
          """
          dispatch.error_log = error_log
          self._st['failed'].append(dispatch)