fix: gracefully handle missing manifest revisions during imports

Closes #654 (closed).

I've double-checked several samples of this error. In all of them, the problem was that there was no entry under docker/registry/v2/repositories/.../_manifests/revisions/sha256/ for the manifest referenced by a given tag. #654 (comment 924863736) is likely the main reason for this data corruption to occur.

In this MR, we simply log and skip the processing of such unlinked manifests.