IDL parser: align with current Web IDL specification

tools/idl_parser/idl_node.py

 #!/usr/bin/env python
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import sys
 
 #
 # IDL Node
 #
@@ skipping 183 matching lines @@
     for child in children:
       if not child:
         continue
       if type(child) == IDLAttribute:
         self.SetProperty(child.name, child.value)
         continue
       if type(child) == IDLNode:
         child._parent = self
         self._children.append(child)
         continue
-      raise RuntimeError('Adding child of type .\n' % type(child).__name__)
+      raise RuntimeError('Adding child of type %s.\n' % type(child).__name__)
 
 
 #
 # Property Functions
 #
   def SetProperty(self, name, val):
     self._properties[name] = val
 
   def GetProperty(self, name, default=None):
     return self._properties.get(name, default)
 
   def GetProperties(self):
     return self._properties