Performs basic validation of field value and then passes it for
        specific validation.

        :param val: field value to validate
        :return: error message or None

def validate(self, val):
        """
        Performs basic validation of field value and then passes it for
        specific validation.

        :param val: field value to validate
        :return: error message or None
        """

        if self.nullable and val is None:
            return
        type_er = self.__type_check(val)
        if type_er:
            return type_er

        spec_err = self._specific_validation(val)
        if spec_err:
            return spec_err