Reply to a query with the QueryFailure flag and an '$err' key.

        Returns True so it is suitable as an `~MockupDB.autoresponds` handler.

def fail(self, err='MockupDB query failure', *args, **kwargs):
        """Reply to a query with the QueryFailure flag and an '$err' key.

        Returns True so it is suitable as an `~MockupDB.autoresponds` handler.
        """
        kwargs.setdefault('flags', 0)
        kwargs['flags'] |= REPLY_FLAGS['QueryFailure']
        kwargs['$err'] = err
        self.replies(*args, **kwargs)
        return True