Registration: wrong URL not detected

[nemesifier]

If a wrong URL is set in /etc/config/openwisp, the register() function does not abort and keeps going ahead.

It should instead abort and report the issue.

[nemesifier]

A possible solution is to use -i option in curl and ensure the HTTP status is 201 before proceeding.

Some changes to the controller are needed anyway.

A possible desired response would be:

HTTP/1.0 201 Created
X-Frame-Options: SAMEORIGIN
Content-Type: text/plain
Connection: close
Server: Werkzeug/0.11.2 Python/3.4.3
Date: Wed, 13 Jan 2016 17:21:40 GMT

registration-result:success
uuid:07ba930c-c7e2-4039-a07b-17ddc4dae317
key:c99rvLRGL3EPzVT4CDo3mT4xAApJu18v