Notification, reminder to back up if scheduled copy not possible

[dnanian]

The Growl notification can be made "sticky", or can use email, so their machine can easily tell them that the backup didn't occur. It'll happen when they're away from the network, at the time of the regular backup.

(Remember, we're not a "Backup server", so it's not a matter of *you* notifying *them* -- their own copy of SD! tells them what happened.)