Improve handling for errors & RFC 9457

[fako1024]

In order to improve the generic handling of RFC 9457 JSON problem responses (and errors in general) the following shall be provided:

• Custom (fallback) error type fulfilling the error interface (to get rid of [body=...] for generic errors
• Improved logic flow for errors
• Actual parsing of the JSON content in a conform structure
• Provide a helper function that can be used outside of httpc, e.g. in conjunction with a default http.Client
• Adjust github.com/els0r/goProbe to use the aforementioned helper function