Bugfix: Improve lock refresh handling after standby

If the restic process was stopped or the host running restic entered standby
during a long running operation such as a backup, this previously resulted in
the operation failing with `Fatal: failed to refresh lock in time`.

This has now been fixed such that restic first checks whether it is safe to
continue the current operation and only throws an error if not.

https://github.com/restic/restic/issues/4274
https://github.com/restic/restic/pull/4374
