feat: support throwing responses in upgrade hook

[LukeHagar]

This PR should introduce support for throwing for errors or returning for success from the upgrade handler, all the interfacing with the adapters should remain the same.

[pi0]

Seems nice direction! It might take me little longer to jump on proper review.

[LukeHagar]

@pi0 Any chance you may be able to take a look at this PR soon?

[LukeHagar]

@pi0 Circling back around on this, I see these changes as a pseudo-prerequisite for getting Web Sockets added to SvelteKit.

I'm happy to work on any changes or feedback that needs to be incorporated and would love to get tests running if some exist.

[LukeHagar]

@pi0 Can you assist with the logic around the errors? I see that the tests expect ws.Error to be populated on unauthorized response, how is that properly set cross platform?

[pi0]

Of course. Applogizes again for delay. I will directly check tests and changes ASAP to move it forward.

[benmccann]

Thanks @pi0! I hope you had a good new year. We're thankful for any time you're able to spare to take a look at this

[pi0]

Thanks again and sorry for the delay. Pushed a few changes:

• Logic for handling hook return value is unified into hooks.upgrade
• Added back partial ResponseInit support for backward compatibility with return { headers }
• If returned Response has ok: false status, it still ends the response also for backward compatibility (discovered from failing tests)