Importer passes on geting tag details error

Context

Currently, when importing tags, the Importer will ignore any error that arises from failure to get the digest from a given tag, moving on to the next tag.

Problem

This could potentially miss manifests that could be imported, but received a transient error, casing a repository to import with incomplete data.

Proposal

Rather than passing on these errors, we should fail the import. This will allow us to complete the strategy described in: #504 (closed)