core: failure to spawn ExecStartPost should not run ExecStop
Failure to spawn ExecStartPost was being handled differently to e.g.
EXIT_FAILURE returned by ExecStartPost. It looks like this was an
oversight. Fix to match documented behaviour.
`man systemd.service`:
> Note that if any of the commands specified in ExecStartPre=, ExecStart=,
> or ExecStartPost= fail (and are not prefixed with "-", see above) or time
> out before the service is fully up, execution continues with commands
> specified in ExecStopPost=, the commands in ExecStop= are skipped.