fix(rest): making non 200 rest errors more informative

[happydave1]

fixes #971

[laskoviymishka]

Direction looks right, and handling the flat S3 Tables {"message":"..."} body makes sense.

I’d request changes before merging though, mainly because this is an error-decoding edge case and currently has no tests. At minimum, please add coverage for:

• flat {"message":"x"} preserving the status sentinel via errors.Is
• canonical {"error":{"message":"x","type":"y"}} still rendering as expected
• empty body / HEAD falling back to the wrapped status error instead of ": "
• the new Error() formatting branches

One small code concern too: the new Code field is decoded but never used. Either include it in the rendered error, or drop it.

Also, e = *payload.Error feels a bit fragile. It would be safer to keep decoding into &e as before and only fill the flat fields when there is no useful nested error.

[laskoviymishka]

Nice!