Handle EIO deadlock when H2 returned an error like 404.

[tmcgilchrist]

Originally the code would only wait on the response and if H2 encountered an error, it would call error_handler instead of response_handler and because error_handler was not defined then errors were sliently ignored.

This fix adds error handling with promise racing and probably addresses this issue #64

Now when an H2 error occurs (like a 404):

1. H2 calls the error_handler instead of hanging
2. The error_handler resolves the error_promise
3. Eio.Fiber.first races the response fiber against the error fiber
4. If it's an error, it gets converted to a proper exception with a descriptive message

[wokalski]

This looks sensible to me 👍