this idea is based on
https://utcc.utoronto.ca/~cks/space/blog/linux/SystemdTimersMailNotes
and the therein linked
https://wiki.archlinux.org/title/Systemd/Timers#MAILTO
but using a top-level systemd override to send such alerts for all
service units on parsons, not just timers. Tested by sending SIGKILL to
monit a couple times & receiving emails.
We might now get two emails for some failing units, or possibly even
three! (if is-system-running is false, caused by a service unit failure,
and monit also notices the service not running). On the other hand, we
now also get emails if monit fails.
