Quieter handling of volumes that are sometimes unavailable

[ewxrjk]

Background: http://www.chiark.greenend.org.uk/pipermail/sgo-software-discuss/2018/000513.html discusses.

It can happen that a volume to be backed up is not mounted when rsbackup runs. check-mounted and check-file can be used to detect this condition, but in some configurations it would be convenient for the volume to be mounted automatically when required, without building policy into rsbackup itself.

pre-backup-hook can be used to do this. But it unavoidably logs on failure, which annoys the user if they already know about the condition (it may be routine) and read their cron mail.

The proposed change is to allow hook scripts to exit with a status indicating a soft failure. By analogy with mail delivery agents the exit status chosen is EX_TEMPFAIL (75).