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

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).