Backdrop sends Bearer token to Stagecraft

The `/data-sets` endpoint is protected by a shared bearer token.
Backdrop must send it when it requests data set metadata.

Quite a lot of test refactoring.

Related to https://www.pivotaltracker.com/story/show/67103208
[#67103208]
Fix `/data-sets`/ URL in fake Stagecraft

Our fake Stagecraft service was responding to /data-sets/ but should
have been responding to /data-sets

Also added logging to make that easier to diagnose.
Add tests for None response and _get_json_url

Fix code to deal with None
Selectively handle HTTP 404 from Stagecraft

Handle an HTTP 404 from Stagecraft only if we requested an invalid
data-set by name.

Stagecraft should never respond with HTTP 404 when we request *all*
data-sets or data-group/data-type combination (instead it would return
an empty list) so we shouldn't swallow that HTTP 404.
Slightly improve the error logging

- Log all HTTP errors in full
- Log the response content (JSON) we got back from Stagecraft