Checks the caveat against all registered caveat conditions.
        :return: error message string if any or None

def check_first_party_caveat(self, ctx, cav):
        ''' Checks the caveat against all registered caveat conditions.
        :return: error message string if any or None
        '''
        try:
            cond, arg = parse_caveat(cav)
        except ValueError as ex:
            # If we can't parse it, perhaps it's in some other format,
            # return a not-recognised error.
            return 'cannot parse caveat "{}": {}'.format(cav, ex.args[0])
        checker = self._checkers.get(cond)
        if checker is None:
            return 'caveat "{}" not satisfied: caveat not recognized'.format(
                cav)
        err = checker.check(ctx, cond, arg)
        if err is not None:
            return 'caveat "{}" not satisfied: {}'.format(cav, err)