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/errors.py

@@ -57,6 +57,13 @@ class ManifestUnknown(V2RegistryException):
   def __init__(self, detail=None):
     super(ManifestUnknown, self).__init__('MANIFEST_UNKNOWN', 'manifest unknown', detail, 404)
 
+class TagExpired(V2RegistryException):
+  def __init__(self, message=None, detail=None):
+    super(TagExpired, self).__init__('TAG_EXPIRED',
+                                     message or 'Tag has expired',
+                                     detail,
+                                     404)
+
 
 class ManifestUnverified(V2RegistryException):
   def __init__(self, detail=None):