Investigate behaviour on partial backup upload

[adamhosier]

The behaviour of failed backup uploading is currently undefined - some backends may leave partial backups around, which the operator will detect as a complete backup and will not retry.

We could checksum the file in the destination rather than simply checking for its existence, but that would involve downloading the entire backup whenever we check if it exists.

We'd like to have defined behaviour for this corner case which is consistent between all storage destinations.