AWS S3 - STS Request Failures come back as Error::Generic

**Describe the bug**
A failure to do a complete AssumeWebIdentity comes back as Error::Generic and not as Error::PermissionDenied
**To Reproduce**
Create an Amazon Client and use STS to authenticate.

**Expected behavior**
The failure should return as Error::PermissionDenied

**Additional context**
Overall, a way to expose the underlying HTTP Response would be extremely helpful when debugging some S3 requests.

Start by reproducing an incomplete AssumeWebIdentity authentication through an Amazon Client using STS and inspect the returned error. Check how the failure is classified and whether the underlying HTTP response can be exposed for debugging; done when the failure is reported as Error::PermissionDenied and the requested response context is available.

Written by the indexing model from the issue text.