core: failure to spawn ExecStartPost should not run ExecStop

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.