Called if the parser encounters an unknown tag.  Tries to fail
        with a human readable error message that could help to identify
        the problem.

def fail_unknown_tag(self, name, lineno=None):
        """Called if the parser encounters an unknown tag.  Tries to fail
        with a human readable error message that could help to identify
        the problem.
        """
        return self._fail_ut_eof(name, self._end_token_stack, lineno)