RestClient::ServerBrokeConnection exceptions cause unhelpful errors

We are getting quite a few RestClient::ServerBrokeConnection exceptions being raised from RestClient

Unfortunately, because all RestClient exceptions were assumed to have valid json responses from the server this resulted in an unhelpful "TypeError: can't convert nil into String". The last commit in this request fixes the issue

I have also done some cleaning up of the spec file itself, reducing the amount of constants being overridden/defined. This helps in preventing cross-test issues later on