Validate
        Perform value validation against validation settings and return
        error object.

        :param value:           list, value to check
        :param model:           parent model being validated
        :param context:         object or None, validation context
        :return:                shiftschema.result.Error

def validate(self, value, model=None, context=None):
        """
        Validate
        Perform value validation against validation settings and return
        error object.

        :param value:           list, value to check
        :param model:           parent model being validated
        :param context:         object or None, validation context
        :return:                shiftschema.result.Error
        """
        invalid = [item for item in value if item not in self.choices]
        if len(invalid):
            return Error(
                self.invalid_multichoice,
                dict(items=', '.join(invalid))
            )

        # success otherwise
        return Error()