IDL: Update type error message for invalid union values

Source/bindings/templates/union.cpp

 // 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.
 
 // This file has been auto-generated by {{code_generator}}. DO NOT MODIFY!
 
 #include "config.h"
 #include "{{header_filename}}"
 
 {% macro assign_and_return_if_hasinstance(member) %}
@@ skipping 115 matching lines @@
 
     {# 18. Boolean (fallback) #}
     {% elif container.boolean_type %}
     {
         impl.setBoolean(v8Value->ToBoolean()->Value());
         return;
     }
 
     {% else %}
     {# 19. TypeError #}
-    exceptionState.throwTypeError("Not a valid union member.");
+    exceptionState.throwTypeError("The provided value is not of type '{{container.type_string}}'");
     {% endif %}
 }
 
 v8::Handle<v8::Value> toV8({{container.cpp_class}}& impl, v8::Handle<v8::Object> creationContext, v8::Isolate* isolate)
 {
     switch (impl.m_type) {
     case {{container.cpp_class}}::SpecificTypeNone:
         {# FIXME: We might want to return undefined in some cases #}
         return v8::Null(isolate);
     {% for member in container.members %}
     case {{container.cpp_class}}::{{member.specific_type_enum}}:
         return {{member.cpp_value_to_v8_value}};
     {% endfor %}
     default:
         ASSERT_NOT_REACHED();
     }
     return v8::Handle<v8::Value>();
 }
 
 {{container.cpp_class}} NativeValueTraits<{{container.cpp_class}}>::nativeValue(const v8::Handle<v8::Value>& value, v8::Isolate* isolate, ExceptionState& exceptionState)
 {
     {{container.cpp_class}} impl;
     V8{{container.cpp_class}}::toImpl(isolate, value, impl, exceptionState);
     return impl;
 }
 
 {% endfor %}
 } // namespace blink