Bugfix: forget command returns exit code 3 on partial removal of snapshots

The `forget` command now returns exit code 3 when it fails to remove one or
more snapshots. Previously, it returned exit code 0, which could lead to
confusion if the command was used in a script.

https://github.com/restic/restic/issues/5233
https://github.com/restic/restic/pull/5322