Bugfix: Improve error handling of repository locking

When the lock refresh failed to delete the old lock file, it forgot about the
newly created one. Instead it continued trying to delete the old (usually no
longer existing) lock file and thus over time lots of lock files accumulated.
This has been fixed.

https://github.com/restic/restic/issues/2452
https://github.com/restic/restic/issues/2473
https://github.com/restic/restic/issues/2562
https://github.com/restic/restic/pull/3512