A file whose data cannot be read or decrypted is left out and listed, and
the command exits with code 2. When the database cannot be restored (its
dump is damaged, or it does not fit this installation), backup:restore:full
leaves the fresh database as it is and puts the data directory and config of
the backup in ngb-salvage-<snapshot> in the data directory. Without --salvage
a restore of damaged data stops as before and points at the option.
