Fix launch errors, improve Pods API, use hook abstraction

Sorry for the largeness of this PR, just took a while to get it all together. @mpuncel @petertseng 

This extracts the preparer's orchestration logic to a package that can be tested. It does not solve the self-hosting issue yet, but gets us most of the way there.
The supervise/ok message is the one we know goes away after a while, however when runit isn't running at all or there's a different error, we want to exit.