Bugfix: Improve handling of temporary B2 delete errors

Deleting files on B2 can sometimes fail temporarily, which requires restic to
retry the delete operation. In some cases the file was deleted nevertheless
causing the retries and ultimately the restic command to fail. This has been
fixed.

https://github.com/restic/restic/issues/3541
https://github.com/restic/restic/pull/3544