Add way to differentiate between websocket upgrade failing and not attempting an upgrade

[adam-fowler]

WebSocket APIs don't provide anyway to differentiate between a websocket upgrade failing and a request not attempting an upgrade. They both go through the notUpgradingCompletionHandler. Ideally I would get some indication of whether a websocket upgrade was attempted and failed so I could return a response indicating the upgrade failure.

Currently when an upgrade failure occurs the pipeline is put into an indeterminate state where some or all of the upgrade HTTP request has been comsumed. See #2632

[Lukasa]

Thanks Adam, this is a good enhancement suggestion.