Improve systemd timer functionality to send email on failure

[gregharvey]

As described here in the Arch Linux docs:

• https://wiki.archlinux.org/title/systemd/Timers#MAILTO

And here again in the OpenSuse docs:

• https://documentation.suse.com/smart/systems-management/html/systemd-working-with-timers/index.html#systemd-mailto

Unlike cron, systemd timers do not have an in-built email mechanism when a job fails. To replicate this we can create a script and service that does just that and use OnFailure in the timer config to trigger it. A simple example is provided in the above documentation.

Note, this would also require a change to the role we use for systemd timers, as it does not currently support adding OnFailure to a timer:

• https://github.com/vlcty/ansible-systemd-timers/blob/master/templates/service.j2#L5