Sets the error on this MultiResult. Will be ignored if an error is
        already set.
        :param exinfo: Return value from ``sys.exc_info()``

def _add_err(self, exinfo):
        """
        Sets the error on this MultiResult. Will be ignored if an error is
        already set.
        :param exinfo: Return value from ``sys.exc_info()``
        """
        if self._err:
            return
        self._err = exinfo
        self.all_ok = False