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.

endpoints/v2/models_pre_oci.py

@@ -37,6 +37,13 @@ class PreOCIModel(DockerRegistryV2DataInterface):
     except database.RepositoryTag.DoesNotExist:
       return False
 
+  def has_tag(self, namespace_name, repo_name, tag_name):
+    try:
+      model.tag.get_possibly_expired_tag(namespace_name, repo_name, tag_name)
+      return True
+    except database.RepositoryTag.DoesNotExist:
+      return False
+
   def get_manifest_by_tag(self, namespace_name, repo_name, tag_name):
     try:
       manifest = model.tag.load_tag_manifest(namespace_name, repo_name, tag_name)