Enhancement: Improve handling of temporary B2 delete errors

Deleting files on B2 could sometimes fail temporarily, which required 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 now been
fixed.

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