Importer: Gracefully Handle Unknown Configuration Blobs

Context

During the course of: &5523 (closed) some imports are failing due to unknown configuration blobs:

1 error occurred:
	* pre importing tagged manifests: pre importing manifest: pre importing manifest: obtaining configuration payload: unknown blob

Problem

This error could either be caused via the layer being unlinked via the Blob Delete API, or from the blob path missing from common blob storage.

Solution

We should investigate the few manifests with this error. While the error does not mention the digest of the blob, we are able to see the digest of the affected manifest, which depends on this config blob.

If the error is cause via the link path not being present (or having zero content), then we should handle this error as we do normal layers and skip the manifest import.

However, if the blob is not present in content addressable storage, this is potentially a more serious issue, and we should take this opportunity to investigate.