Raise AbsentLinkSecret if link secret is not set.

        :param action: action requiring link secret

def _assert_link_secret(self, action: str):
        """
        Raise AbsentLinkSecret if link secret is not set.

        :param action: action requiring link secret
        """

        if self._link_secret is None:
            LOGGER.debug('HolderProver._assert_link_secret: action %s requires link secret but it is not set', action)
            raise AbsentLinkSecret('Action {} requires link secret but it is not set'.format(action))