logind: add missing resume signal when we fail to initiate sleep/shutdown

This fixed https://bugzilla.redhat.com/show_bug.cgi?id=1476313
as much as I was able to reproduce it in a VM, at least.

E.g. this signal might wake the screen back up, providing a more visible
indicator of suspend failure.  In my VM testing, it was also required in
order to unblock keyboard input in gnome-shell after the failed suspend.

At the same time, fix the error handling for scheduled shutdowns.  This now
mirrors the behaviour of when you use `shutdown -k` - it sends all the
scary messages about shutting down, "but you'll have to do it [shut down
the system] yourself".  It also avoids the risk of locking out the admin
(nologin file), in case they logged out for some reason (and they use
`sudo` instead of root).

Not that I have any idea why you'd want to use `shutdown -k`, but the code
is easier to analyze if it rolls back on error (in the absence of any code
comment as to why that's not wanted).

(cherry picked from commit 6d7f7fd4)