Bugfix: Fix rare failures to retry locking a repository

Restic 0.17.0 could in rare cases fail to retry locking a repository if one of
the lock files failed to load, resulting in the error:

```
unable to create lock in backend: circuit breaker open for file <lock/1234567890>
```

This issue has now been addressed. The error handling now properly retries the
locking operation. In addition, restic waits a few seconds between locking
retries to increase chances of successful locking.

https://github.com/restic/restic/issues/5005
https://github.com/restic/restic/pull/5011
https://github.com/restic/restic/pull/5012
