logind: do not propagate error in delayed action

If the action failed, we should log about the issue, and continue.
Exiting would bring the graphical session down, which of course is not
appreciated by users.

As documented in previous commits, a non-negative return from the callback
doesn't matter, so the callback is simplified a bit.

Fixes #21991.

(cherry picked from commit 8207b832)
(cherry picked from commit fb9bbbee)
(cherry picked from commit e6ca5aa8)