For automated backups, retry with backoff on error

I have backups on Google Drive. When my Mac reboots, it starts up both Drive and Scythebill in tandem. If Scythebill comes up first and tries to backup immediately, I get a warning of a failed backup.

Retrying after (say) 5 minutes would entirely address that problem, but more generally not erroring out immediately and hoping (for example) for a drive to get mounted or an internet connection to come back would make things a bit more reliable.