Importer passes on manifest list reference errors

Context

Currently, when importing child manifests, the Importer will ignore any error that arises from failure to get the child manifest, import it, or associate it with the parent manifest.

Problem

This could potentially miss manifests that could be imported, but received a transient error, causing 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)