HTTP reponse errors raise ProtocolError, but should raise some HTTPError with the actual status code

[SpComb]

Client code might be interested in handling specific websocket handshake HTTP errors like 404, 403 etc differently. These currently just result in generic human-readable messages like raise Kontena::Websocket::ProtocolError, "Error during WebSocket handshake: Unexpected response code: 404", which are clumsy to deal with.

Looks like the Websocket::Driver::Client will set the http @status and @headers before failing the handshake and emitting the error.

The same error also gets emitted on things like Invalid HTTP response, though.