docs: def error(self):
          """
          Return an instance of Exception if any, else None.
  
          Actually check for a :class:`TimeoutError` or a
          :class:`ExitCodeError`.
          """
          if self.__timed_out:
              return TimeoutError(self.session, self, "timeout")
          if self.__exit_code is not None and \
                  self.__expected_exit_code is not None and \
                  self.__exit_code != self.__expected_exit_code:
              return ExitCodeError(self.session, self,
                                   'bad exit code: Got %s' % self.__exit_code)