Backup agent script can silently die

Happened when running a `mysql` dump type backup

Culprit appears to be https://github.com/ppp0/puppet-packages/blob/master/modules/backup/templates/agent/rdiff/create.sh#L45 which can break the script with no output - meaning that `cron` will not send any mails

Idea:
- add a trap to catch exit != 0's