general error handling (no answer from backend, failed to parse answer from backend)

[KirDE]

In the client, we should just have a general exception handler that shows an error message. (For alle pages in App.tsx) (React Error Boundary component) (@NineBerry )

[NineBerry]

Need a design how an error page should look like

[KirDE]

for now just use error styles for new banner

[fauri13]

One thing is the ErrorBoundary component, that catches unhandled errors in the code and another thing is to handle api errors.
Right now, errors from API are already catched and do not throw a fatal error. I suggest leaving it as is for now as I don't thinks is an issue right now. Haven't heard any complains about it and it's difficult to test. I'm not saying we should do nothing but maybe move to a future version as it can take some time to do it right.