'Check whether a username is valid.'

@csrf_exempt
@json_validate(user=VUname(('user',)))
def POST_check_username(self, responder, user):

    if (not responder.has_errors('user', errors.USERNAME_TOO_SHORT, errors.USERNAME_INVALID_CHARACTERS, errors.USERNAME_TAKEN_DEL, errors.USERNAME_TAKEN)):
        return {}
