suggestion: ship mail_on_failure separate from systemd-cron package

[Mark Stosberg]

Hello,

I'd like to write my own systemd timers, but would like an easy way to
send mail on failure. Splitting out /lib/systemd-cron/mail_on_failure
from the systemd-cron package would help with that.

Right now, installing systemd-cron on 16.04 causes ubuntu-standard and
other things to be removed if I don't want, so installing systemd-cron
just to get the `mail_on_failure` tool is not a good option.

The source code for the script is here:
https://github.com/systemd-cron/systemd-
cron/blob/master/src/bin/mail_on_failure.py

There's one known bug for it here:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=792777

There's also an open ticket about the the issue in Launchpad, where I
was advised to redirect the suggestion here:
https://bugs.launchpad.net/ubuntu/+source/systemd-cron/+bug/1583743

Thanks,

     Mark Stosberg