support reporting errors from workers to worker-manager

[imbstack]

Discussion of what to do with this is in https://bugzilla.mozilla.org/show_bug.cgi?id=1562285

[imbstack]

(requires g-w-protocol and protocol-startup)

[djmitche]

I believe this is no longer blocked, as generic-worker now uses the protocol and it is initialized properly at startup.

[milescrabill]

The outstanding bit here is the docker-worker portion.

[lotas]

Looks like worker-runner is already doing this, g-w not yet? @petemoore do you think it should be?