GitRef attempts to decode invalid results, inscruitable error

[Stuart Bishop]

GitRef calls gitRefFromLink, which is getting a successful (err == nil) result from Launchpad. But the body is not JSON, and the decode fails with the useless-for-further-diagnosis error: failed to unmarshal gitref: invalid character '<' looking for beginning of value

We should catch non-JSON responses and give a more useful error message. Suspect we are getting a login or permission denied HTML page.