Improve error reporting on a failed backup slightly

Whenever there's a failed backup, make sure we print the host/filesystem being
backed up. This information is logged, and if you have debug mode on it will
print the filesystem being backed up before any error, but a production run
via cron would just show '=> failure zfs_full_backup: failed error: 65280 at
/opt/omni/bin/zetaback line 728.'.