Enhancement: Improve messages `restic check` prints

Some messages `restic check` prints are not really errors, so from now on
restic does not treat them as errors any more and exits cleanly.

https://github.com/restic/restic/pull/1709
https://forum.restic.net/t/what-is-the-standard-procedure-to-follow-if-a-backup-or-restore-is-interrupted/571/2
