Upload refactoring

When uploading a program, and something went wrong on the web, a JSON object with statusCode and message is returned.
But we did not check the statusCode, and got "Upload successful" even when the program was not stored on the web.
This is just a quickfix - we should give meaningful errormessages; currently the same toast is displayed no matter what went wrong on the server.

successful testrun: http://catrobatgw.ist.tugraz.at:8080/job/Catroid-single-UI-test/386/
(japanese test was removed temporarily because it did not work since change to FTP upload - but it did always pass, because we didn't check the statusCode :D)