Re-raise 4xx responses from server when uploading files (#284)

Fixes #270

We were falling back to base64 encoding the file data when requests to
upload the file failed. However if the client makes an invalid request
such as failing to include Authorization headers by forgetting to auth
then we should surface these errors.

This commit now re-raises any 4xx errors returned while attempting to
upload a file and adds tests to verify the behavior.

Author: aron