Improve handling of error responses from the API

[spiritualized]

Fixes #

Changes proposed in this pull request:

• Raise a WSError if the API response contains a non-200 code, instead of failing to interpret the body and raising a MalformedResponseError

[spiritualized]

@hugovk, will you accept this change?

I'm not getting the test failures locally, perhaps there's a problem with the configured password or API key.

[hugovk]

Yeah, tests fail for PRs due to #171.

I was wondering, not every non-200 status is an error. For example, could there be 3xx redirect statuses? How about other statuses?

[spiritualized]

Good point. Altered the check to only catch relevant 5xx errors.

[hugovk]

Thank you!