add failed insert cleanup

mongo/collection.go

@@ -227,6 +227,8 @@ func (self *Collection) Append(item interface{}) (interface{}, error) {
 
 		// Now append data the user wants to append.
 		if err = self.collection.Update(bson.M{"_id": id}, item); err != nil {
+			// Cleanup allocated ID
+			self.collection.Remove(bson.M{"_id": id})
 			return nil, err
 		}
 	}