Do not eagerly convert exception to string when creating a message object

test/cctest/test-api.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4780 matching lines @@
       "ReferenceError.prototype.constructor.name = 1;"
       "Number.prototype.toString = function() { return 'Whoops'; };"
       "ReferenceError.prototype.toString = Object.prototype.toString;");
   CompileRun("asdf;");
   v8::V8::RemoveMessageListeners(check_reference_error_message);
 }
 
 
 static void check_custom_error_tostring(v8::Handle<v8::Message> message,
                                         v8::Handle<v8::Value> data) {
-  const char* uncaught_error = "Uncaught MyError toString";
+  const char* uncaught_error = "[object Object]";
   CHECK(message->Get()->Equals(v8_str(uncaught_error)));
 }
 
 
 TEST(CustomErrorToString) {
   LocalContext context;
   v8::HandleScope scope(context->GetIsolate());
   v8::V8::AddMessageListener(check_custom_error_tostring);
   CompileRun(
       "function MyError(name, message) {                   "
@@ skipping 16305 matching lines @@
   // add the testExtraShouldReturnFive export
   v8::Local<v8::Object> exports = env->GetExtrasExportsObject();
 
   auto func =
       exports->Get(v8_str("testExtraShouldReturnFive")).As<v8::Function>();
   auto undefined = v8::Undefined(isolate);
   auto result = func->Call(undefined, 0, {}).As<v8::Number>();
 
   CHECK(result->Value() == 5.0);
 }