Make the v2 logic fallback on v1 when v2 requests cannot be authorized.

Signed-off-by: Matt Moore <mattmoor@google.com>

graph/pull.go

@@ -482,6 +482,10 @@ func (s *TagStore) pullV2Repository(r *registry.Session, out io.Writer, repoInfo
 	if err != nil {
 		return fmt.Errorf("error getting authorization: %s", err)
 	}
+	if !auth.CanAuthorizeV2() {
+		return ErrV2RegistryUnavailable
+	}
+
 	var layersDownloaded bool
 	if tag == "" {
 		logrus.Debugf("Pulling tag list from V2 registry for %s", repoInfo.CanonicalName)