Importer passes on any manifest import error when importing tags

Context

Currently, when importing tags the Importer will ignore any error that arises from a failed manifest import.

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)