[DirectBinaryAccess] Verify that binary exists in cloud before creating signed download URI

The direct binary access download logic doesn't actually verify that the requested blob is available in the cloud before creating the signed download URI.  It is possible that a user could request a download URI for a blob that is "in the repo" but hasn't actually been uploaded yet.

We should verify this by uploading a new blob, preventing it being uploaded to the cloud (retain in cache), and then request the download URI.  We should get a null back or get some other error or exception; if we get a URI it would return an HTTP 404 if the blob is not actually uploaded yet (maybe this would also be ok).