Proxy should not respond with `200 OK` when local server is gone

[ludgerey]

Steps to produce:

• zrok share public http://localhost:8888 (or any other port)
• Make sure there is nothing running on that port
• Make HTTP call (e.g. curl --location 'https://ubm7ws4tsykj.share.zrok.io/foo')

Expected result:

• It should return a 502 Bad Gateway or another error code

Actual result:

• It returns 200 OK and an empty body

This is especially annoying if your local server works initially but then crashes and you still get successful HTTP responses, but the content not what you expect.

[michaelquigley]

Will try and see what we can do here for the next release.