Fix duplicate error check

mongoengine/document.py

@@ -280,7 +280,9 @@ class Document(BaseDocument):
                     kwargs.update(cascade_kwargs)
                 kwargs['_refs'] = _refs
                 self.cascade_save(**kwargs)
-
+        except pymongo.errors.DuplicateKeyError, err:
+            message = u'Tried to save duplicate unique keys (%s)'
+            raise NotUniqueError(message % unicode(err))
         except pymongo.errors.OperationFailure, err:
             message = 'Could not save document (%s)'
             if re.match('^E1100[01] duplicate key', unicode(err)):