Change error message when trying to pull a deleted or expired tag

Will let the users know they can recover the tag via time machine

Note: This was tested with the Docker protocol, but the new error code is *technically* out of spec; we should make sure its okay.

data/model/tag.py

@@ -530,6 +530,15 @@ def get_active_tag(namespace, repo_name, tag_name):
                            Namespace.username == namespace)).get()
 
 
+def get_possibly_expired_tag(namespace, repo_name, tag_name):
+  return (RepositoryTag
+          .select()
+          .join(Repository)
+          .join(Namespace, on=(Repository.namespace_user == Namespace.id))
+          .where(RepositoryTag.name == tag_name, Repository.name == repo_name,
+                 Namespace.username == namespace)).get()
+
+
 def associate_generated_tag_manifest(namespace, repo_name, tag_name, manifest_digest,
                                      manifest_data):
   tag = get_active_tag(namespace, repo_name, tag_name)