IDL: Use user-friendly type names for error messages

Source/bindings/scripts/idl_types.py

 # Copyright 2014 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.
 """IDL type handling.
 
 Classes:
 IdlTypeBase
  IdlType
  IdlUnionType
  IdlArrayOrSequenceType
@@ skipping 235 matching lines @@
 
 class IdlUnionType(IdlTypeBase):
     # http://heycam.github.io/webidl/#idl-union
     # IdlUnionType has __hash__() and __eq__() methods because they are stored
     # in sets.
     def __init__(self, member_types):
         super(IdlUnionType, self).__init__()
         self.member_types = member_types
 
     def __str__(self):
-        return self.name
+        return '(' + ' or '.join(str(member_type) for member_type in self.member_types) + ')'
 
     def __hash__(self):
         return hash(self.name)
 
     def __eq__(self, rhs):
         return self.name == rhs.name
 
     def __getstate__(self):
         return {
             'member_types': self.member_types,
@@ skipping 128 matching lines @@
     def is_nullable(self):
         return True
 
     @property
     def name(self):
         return self.inner_type.name + 'OrNull'
 
     def resolve_typedefs(self, typedefs):
         self.inner_type = self.inner_type.resolve_typedefs(typedefs)
         return self