Raise exception on failed http request

[michaelwood]

If the http server doesn't return OK then raise an exception. This is especially useful for debugging incorrect remote references rather than just getting json encoding errors.

[coveralls]

Coverage decreased (-0.3%) to 91.262% when pulling 57299e9dfaef0a7e3f770d97a01e02155ca75b06 on michaelwood:master into 066132e527f8115f75bcadfd0eca12f8973a6309 on gazpachoking:master.