Resolve "Fix nullpointer from http status code -1"

Changes in this merge request

• Callback method onResult returns a boolean. Returns false if the status code is not accepted and the request should be retried. Returns true if everything is fine.
• Callback methods in all relevant controllers are adapted to the mentioned change in return type. All now return false if the status code is -1.
• HTTPResponseHandler calls the failed method if the callback onResult method returns false.

Closes #45 (closed)