Improve publish command error handling
The OpenStore does not respond with JSON on errors. We try to parse it as json though. That is where the cryptic
Expecting value: line 1 column 1 (char 0) messages come from. This MR makes the response text being printed instead, which is HTML and therefore also not optimal. See https://github.com/SuperTux/supertux/runs/2160975168?check_suite_focus=true for example.