l2 cache backups survive clean up

[j-haacker]

Describe the bug
Sometimes l2 cahche files survive after the function ends. However, to protect from inconsistent data if there is an existing backup an error is raised (it is assumed another process works on the same data). This is inconvenient and should be fixed.

To Reproduce
I don't exactly know. I believe this occurs if the function ends because of external kill signals (like the job's computation time expires in a scheduling system).

Expected behavior
If the function ends regularly, the backup should be removed. If it is killed, the backup might need to be employed - if this succeeds, it should be removed afterward.