Honor sync errors

Fractal does not take notice of errors in response to sync request. They go under the radar because accidentally the JSON error response gets just parsed into an empty SyncResponse object and is handled as if no error occurred. This PR checks the HTTP status code to see if really everything’s all right and returns an error if it’s not.

I'd appreciate help on how to proceed with Matrix error codes in the Fractal way as I have no overview over how they are handled in Fractal. In its current form my code simply tosses an error to the console – so barely real error handling. What is Fratal’s design of handling those errors?