Better error messages

Fixes #698 's subtask #654 

The correct message from server is printed out.
statusCode from server is logged, not printed anymore. The user only gets a meaningful message what went wrong.

Jenkins:
old: https://jenkins.catrob.at/job/Catroid-Multi-Job-Custom-Branch/1480/
new: pending-> https://jenkins.catrob.at/job/Catroid-Multi-Job-Custom-Branch/1499/