Better error message when using non-existant presets or using split presets wrong

[zyzzyxdonta]

Thank you for the great tool! I've been using crestic for years now. However, there's one small issue that I ran into a number of times. I usually run crestic via a systemd unit and this comes up whenever I haven't used it manually in a while and don't quite remember my setup:

$ crestic home backup
Fatal: nothing to backup, please specify source files/dirs

The issue here is that home doesn't exist as a preset. It's actually a split preset home@ and a correct preset name would be home@nas.

The error message shown here is from restic. I think it would be a great improvement to check the preset name passed to crestic and show an error message that explains the issue better.

Thanks!