return error if `member` is not a member of any class in
        `classes`

def _validate_member(self, member, classes, instanceof):
        """return error if `member` is not a member of any class in
        `classes`
        """
        log.info("Validating member %s" % member)

        stripped = self._get_stripped_attributes(member, classes)
        if self._field_name_from_uri(member) in stripped:
            all_class_members = sum([self.schema_def.attributes_by_class[cl]
                                     for cl in classes], [])
            if member in all_class_members:
                return
            if self._namespace_from_uri(member) in self.allowed_namespaces:
                err = self.err("Unoficially allowed namespace {0}",
                               self._namespace_from_uri(member))
                return ValidationWarning(ValidationResult.WARNING, err['err'],
                                         err['line'], err['num'])
        else:
            err = self.err("{0} - invalid member of {1}",
                           self._field_name_from_uri(member),
                           self._field_name_from_uri(instanceof))
            return ValidationWarning(ValidationResult.ERROR, err['err'],
                                     err['line'], err['num'])