[Plan] Send emails

When cron updates go into state where there are likely not succeed anymore. This could because the installed branch is unsupported or readiness checkers start to throw errors(maybe warnings?)

This seems like a good idea on its face, but how do we prevent it from becoming a torrent of spammy emails? I guess we could set a state flag that says "we entered an invalid readiness state, and warned about it", clear that flag when the readiness checks come back green, and not send the email unless the last known state was good...

Of the stuff I see in the issue summary right now, I'd say that our only actual hard stable blocker, right now, is to send an email when a cron update fails. The rest are nice-to-haves, maybe even critical post-stable features, but I don't personally think they block a stable release.

Our first stable release need not include email support, so this is no longer a stable blocker. See #3284443: Enable unattended updates.