Add better handling internal and remote errors

[mosiac1]

Discussion originally started from: #153.

aws-proxy used to only forward the bodies of 2XX responses, otherwise sending just the headers. This was changed by the PR above to always forward the bodies, regardless of the status code.

Additionally, if an error is raised internally in aws-proxy (e.g., a server error causing a 500 or an access control check failing causing a 403) these send HTML bodies.

Most S3 clients do expect errors to have an XML body with more details but can also handle missing or badly formatted bodies (the status code is enough to raise an exception). Having no details for errors does make it harder to debug.