Hookshot should crash if homeserver refuses to accept the AS token

[jaywink]

If for some reason the homeserver is reconfigured without the token used by Hookshot, there should be a crash to alert sysadmins of an issue. Currently it seems Hookshot will happily run but wont be able to talk to a homeserver.

[Half-Shot]

For HS -> AS traffic, I'd expect the homeserver to have a metric for failing traffic that we should catch.

For AS -> HS traffic, yeah there should be an easy hook for when Appservice can't auth, and that's something we can gracefully exit on. (Restarted hookshots are unhealthy until they successfully make a request).