remove positional arguments support for Net::HTTP.
always raise a Roar::Transport::Error exception for NetHttp when the status code is not 2xx. this kinda fixes #109 for now, although i'd love to have @paulccarey's more generic error mapping for both NetHTTP and Faraday.

the raised exception contains the response via #response which in turn is simply the original NetHTTP exception, making the following work.

exception.response.code

if you would like to have this API directly on exception (Error#code) feel free to PR. :heart: