We should somehow handle a case where blob for which we want to fetch size, doesn't exist (e.g. it was removed meanwhile).
Updated the sizes endpoint to return 0 size for such blobs.
Also, slightly refactored S3 error handling to extract the NotFound check to a dedicated func.

Depends on D14649

tested on staging. Sizes endpoint returns actual size for existing blobs and 0 for non-existing.