IDL: Update type error message for invalid union values

Source/bindings/scripts/v8_union.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.
 
 import v8_utilities
 
 
 UNION_H_INCLUDES = frozenset([
     'bindings/core/v8/Dictionary.h',
     'bindings/core/v8/ExceptionState.h',
@@ skipping 91 matching lines @@
         raise Exception('%s contains more than one nullable members' % union_type.name)
     if dictionary_type and nullable_members == 1:
         raise Exception('%s has a dictionary and a nullable member' % union_type.name)
 
     return {
         'array_buffer_type': array_buffer_type,
         'array_buffer_view_type': array_buffer_view_type,
         'boolean_type': boolean_type,
         'cpp_class': union_type.cpp_type,
         'dictionary_type': dictionary_type,
+        'type_string': str(union_type),
         'includes_nullable_type': union_type.includes_nullable_type,
         'interface_types': interface_types,
         'members': members,
         'needs_trace': any(member['is_traceable'] for member in members),
         'numeric_type': numeric_type,
         'string_type': string_type,
     }
 
 
 def member_context(member, interfaces_info):
@@ skipping 12 matching lines @@
             cpp_value='impl.getAs%s()' % member.name, isolate='isolate',
             creation_context='creationContext'),
         'is_traceable': member.is_traceable,
         'rvalue_cpp_type': member.cpp_type_args(used_as_rvalue_type=True),
         'specific_type_enum': 'SpecificType' + member.name,
         'type_name': member.name,
         'v8_value_to_local_cpp_value': member.v8_value_to_local_cpp_value(
             {}, 'v8Value', 'cppValue', isolate='isolate',
             needs_exception_state_for_string=True),
     }