Small bug on run_validation

[papagala]

This does not work since lex is not defined in this scope

    def run_validation(self, validators):
        for error in validators:
            if error:
                return error

        return lex.delegate()

This will work

    def run_validation(self, validators):
        for error in validators:
            if error:
                return error

        return self.delegate()