[backup-client][native_rust_library] update file cleanup to ignore missing files

Summary:
ENG-9656.

Right now, it's possible to upload only User Keys, and we don't want for cleanup to fail.

Depends on D13928

Test Plan: Make sure cleanup is not failing when there is no User Data

Reviewers: bartek, tomek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13929