Raise a 409 if we try to insert a tag twice at the same time

Also fixes handling of labels for existing manifests

Fixes #1775

endpoints/v2/errors.py

@@ -99,6 +99,13 @@ class SizeInvalid(V2RegistryException):
                                       detail)
 
 
+class TagAlreadyExists(V2RegistryException):
+  def __init__(self, detail=None):
+    super(TagAlreadyExists, self).__init__('TAG_ALREADY_EXISTS',
+                                           'tag was already pushed',
+                                           detail,
+                                           409)
+
 class TagInvalid(V2RegistryException):
   def __init__(self, detail=None):
     super(TagInvalid, self).__init__('TAG_INVALID',