attempt to refetch proxy repository content from remote when a referenced local blob is missing

Background
In the case of a damaged/ill-timed blobstore backup/restore, it's possible there are some missing blobs.

Acceptance

• If a blob in a proxy repo is missing, rather than immediately returning a 500 error, Repo attempts to refetch it.
• If the content is available in the remote, it is downloaded.

Notes
We should ensure we're only trying this for 'missing blob' situations that correspond to proxied artifacts. If it's a missing blob for cached metadata, there will be nothing to refetch.