Throw error if request is not successful (non 200)

There is currently no way to know whether or not a request failed. A resource only ever returns its body, never any information about the response headers.

Should Argonaut throw an error if the request is deemed unsuccessful? This is exactly how Angular does it in `$http`. If not, have you got any other ideas about how this information should be passed through?

https://github.com/angular/angular.js/blob/cceb455fb167571e26341ded6b595dafd4d92bc6/src/ng/http.js#L747