Bugfix: Improve error handling of repository locking

Previously, 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 now 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
