core: go to failure state if the main service process fails and RemainAfterExit=yes...

core: go to failure state if the main service process fails and RemainAfterExit=yes (#9159)

Previously, we'd not care about failures that were seen earlier and
remain in "exited" state. This could be triggered if the main process of
a service failed while ExecStartPost= was still running, as in that case
we'd not immediately act on the main process failure because we needed
to wait for ExecStartPost= to finish, before acting on it.

Fixes: #8929