Backup with attachments fails if one or more S3 files are missing

Starting a backup that includes attachments may fail if some items are no longer available. The backup file is not produced.

Steps to Reproduce:

1. Run a backup with attachments when some files referenced in the instance no longer exist in S3.
2. The process fails before completion.

Expected Behavior: The backup should complete successfully. If certain items are unavailable, they should be skipped so the backup can still be created (optionally informing the user about any omissions).

Actual Behavior: The backup fails entirely when one or more items are unavailable.