Be more flexible with HTTP response codes.

post-review's HTTP GET/POST code always assumed that any Review Board API
error code would be in an HTTP 200, and that anything else was an unknown
failure. It's now more flexible and understands that any HTTP status code
could have a JSON payload in it, and processes them accordingly.

This introduces some new unit tests for testing the error parsing code
paths.

Ran the new unit tests, and also posted this review request.