Borg 1.2 exits 0 for success, exists 1 for backup completion with
warnings and exits >1 for proper errors. The most common cause of a
warning appear to be a file changing while it is backed up. This causes
paste02 backups to consistently email us because our borg backup script
exits 1 which we treat as a failure.
Update the script so that when borg 1.2.8 is in use we treat rc 1 as a
success with warnings rather than failure with warnings. This should
make our cron job quieter and match the old 1.1.18 behavior.
To better test this we also drop the backup exclusion for the borg
backup log file (which we write to as we backup so it is very likely to
change during backups).
Change-Id: Iab69f0d5951247897d204dcb0a2face424472db0
37a8b1e9d0