Bugfix: Ignore/remove invalid lock files

This corrects a bug introduced recently: When an invalid lock file in the repo
is encountered (e.g. if the file is empty), the code used to ignore that, but
now returns the error. Now, invalid files are ignored for the normal lock
check, and removed when `restic unlock --remove-all` is run.

https://github.com/restic/restic/issues/1652
https://github.com/restic/restic/pull/1653
