Do not have an attr_reader for "message"

Holy fuck - this is quite possibly the hardest bug I have ever squashed
with the simplest solution. Remove this one line.

By setting an attr_reader for :message, we have defined the message
method on the error. However, superclass defines message as an alias to
#to_s, so we completely broke that chain. #oops

Fixes #25
Use chunked transfer for uploads
Improve error swallowing of non-JSON responses
Fix the APIs for uploading artifacts
Update tests to reflect new method airity