Display the specific error in the case of a 403

[rubyist]

A status of 403 against the API would be masked with a generic message. This displays a more specific message - either one returned in the API json, or a default message about authentication.

[technoweenie]

Should we just do this with every status code that falls through? I think returning the err would be better than losing it here:

    if res.StatusCode != 200 {
        return nil, Error(fmt.Errorf("Invalid status for %s %s: %d", req.Method, req.URL, res.StatusCode))
    }

Assuming that the response has valid json with a "message" property, the error should get this info here.

[rubyist]

Yeah that makes sense. That latest commit handles all the fallthroughs. The changed test shows that if the call returns some weird status without json it'll display an error about being unable to parse the response.