Bugfix: Return exit code 3 when failing to remove snapshots

Previously, the `forget` command returned exit code 0 when it failed
to remove one or more snapshots. This was misleading to scripts.

The `forget` command now instead returns exit code 3 when failing to
remove one or more snapshots.

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