[suggestion] Provide clearer error messages for client-side authentication failures (4xx)

[LeoCBS]

Hello,

It seems that the auth crate doesn't properly parse response errors when the authentication configuration is invalid.
I was trying to use GKE Workload Identity as the auth method behind the with_auth() function, and the crate just returned a decode error (probably because the response wasn't what the decoder expected), which made debugging my configuration more difficult.

What do you think about returning a clearer error message for client auth errors? Does that make sense?
I’d be happy to open a PR with this suggestion if you're open to it.

HttpError(reqwest::Error { kind: Decode, source: Error("expected value", line: 1, column: 1) })

Just to clarify, Workload Identity on GKE works fine, this suggestion is only about improving the clarity of error messages in such cases.

[yoshidan]

I agree with including the request URL and bytes of the response body in the error, as it is difficult to investigate whether that error actually occurred.