Wrap username validator for WTForms.

def validate_username(form, field):
        """Wrap username validator for WTForms."""
        try:
            validate_username(field.data)
        except ValueError as e:
            raise ValidationError(e)

        try:
            user_profile = UserProfile.get_by_username(field.data)
            if current_userprofile.is_anonymous or \
                    (current_userprofile.user_id != user_profile.user_id and
                     field.data != current_userprofile.username):
                # NOTE: Form validation error.
                raise ValidationError(_('Username already exists.'))
        except NoResultFound:
            return