Host fixes

There are two fixes here:

1) ensure that job errors which aren't `ExitError` get propagated by the attach protocol (this is the reason why the deploys succeed in #402 even though the job actually failed with `Unable to set up networking: netlink rib: bad file descriptor`)
2) ensure a failed init process (e.g. due to not being able to setup the network) exits straight away
Is there any reason to use `exitCode` here? It looks like the definition could be moved down to where it is assigned to the result of `babySit` and this could use a constant.