Proxy should return informational errors when authorization fails

[vagaerg]

At the moment, any invalid authorization will return a 403 error with no meaningful description. There is no way for the caller to know whether the request failed because of a malformed/illegal authorization or a validly formatted but unauthorized one.

The S3 API includes specific error statuses for each of these cases - https://docs.aws.amazon.com/AmazonS3/latest/API/ErrorResponses.html

As originally discussed in #11 - #11 (comment) - this should be implemented in order to retain full S3 compatibility