Throw exceptions for failure status codes with more data about the response in them.

Some APIs return useful messages and other data in 401 responses, etc, but `HttpResponseMessage.EnsureStatusCode()` eats the response, and only includes the status code in the _message_ of the exception it throws.

This PR adds (and throws) a custom exception type with enough information in it to deal with the error properly in calling code.

_I was able to achieve this in my own project using a custom message handler that intercepts the response and throws a custom exception, but it feels like a lot of work and a bit of a hack to have to do that._
@bennor Yep, that's what I would do - the failure content is just bonus points, we can tolerate if it fails