Add status code to exceptions

[mfrister]

Currently, Grac Exceptions don't contain the HTTP response code, so just forwarding the error in a response is not possible, but rather requires manually mapping exceptions back to status codes.

Also, for generic ServiceExceptions, there's no way to access the response code.

Grac ClientExceptions should contain the HTTP response code, so we can access it.

Make sure to not break the existing API - when adding a parameter to the ClientException initializer, make it optional.