Send session_rollback signal in sqlalchemy ``after_rollback``.

        This marks the failure of session so the session may enter commit
        phase.

def session_rollback(self, session):
        """Send session_rollback signal in sqlalchemy ``after_rollback``.

        This marks the failure of session so the session may enter commit
        phase.
        """
        # this may happen when there's nothing to rollback
        if not hasattr(session, 'meepo_unique_id'):
            self.logger.debug("skipped - session_rollback")
            return

        # del session meepo id after rollback
        self.logger.debug("%s - after_rollback" % session.meepo_unique_id)
        signal("session_rollback").send(session)
        self._session_del(session)