Understand why some files aren't transfered in the GCP backup

In https://github.com/rust-lang/infra-team/issues/122 we created the backup but some files failed to be transferred (0.x%).

More details in [#t-infra > s3-tsv @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/s3-tsv/near/541407443).

Try to understand why these files are not being transferred and fix the issue.