GCS backend - More granular logging depending on GCS response codes

[dcaba]

Right now, at least in the GCS backend implementation:

sccache/src/cache/gcs.rs

Line 519 in 65187dd

Err(e) => {

There are only two situations from an error management perspective:

• We found a cached object in the bucket and we are getting the associated data, so great...
• Everything else, that triggers a WARN in the logs/output, if enabled

Problem is that everything else...

• We are generating WARN on 404 responses, which is very legit when we are building changes/new things, generating noise / even misunderstandings
• Auth issues are also logged as WARN (but they effectively make the backend unusable), rather than ERROR, which are probably very relevant, and easy to miss depending on log level (and the rest of noise now we categorize as WARN)

Should we inspect the inner BadHttpStatusError and segregate things like this? Happy to submit an small PR about that, if that makes sense to others.