Backup.sh should return error code on failures

[nalt]

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If the backup fails, backup.sh returns an error code of 0. Instead, it should return an error code.

In case of failed backups, the following script rotate.sh should probably not remove any old backups, since it cannot detect corrupted ones. It should rather wait until the backup works again.

[woolfg]

thank @nalt for the observation. full ack here as well, do you want to contribute a fix for it? cheers, Wolfgang