Bugfix: Yield error on invalid policy to `forget`

The `forget` command previously silently ignored invalid/unsupported
units in the duration options, such as e.g. `--keep-within-daily 2w`.

Specifying an invalid/unsupported duration unit now results in an error.

https://github.com/restic/restic/issues/3861
https://github.com/restic/restic/pull/3862
