Return MaybeHandle from Invoke.

src/messages.cc

Index: src/messages.cc
diff --git a/src/messages.cc b/src/messages.cc
index 0136b45c4bd45b18f73d62b3ad605cb1fd618c52..776ac8a4366d041b9d07b66d230d72cd7eef1ad7 100644
--- a/src/messages.cc
+++ b/src/messages.cc
@@ -161,13 +161,12 @@ Handle<String> MessageHandler::GetMessage(Isolate* isolate,
   Handle<Object> argv[] = { Handle<Object>(message->type(), isolate),
                             Handle<Object>(message->arguments(), isolate) };
 
-  bool caught_exception;
-  Handle<Object> result =
-      Execution::TryCall(fun,
-                         isolate->js_builtins_object(),
-                         ARRAY_SIZE(argv),
-                         argv,
-                         &caught_exception);
+  bool caught_exception = false;
+  Handle<Object> result = Execution::TryCall(fun,
+                                             isolate->js_builtins_object(),
+                                             ARRAY_SIZE(argv),
+                                             argv,
+                                             &caught_exception);
 
   if (caught_exception || !result->IsString()) {
     return factory->InternalizeOneByteString(STATIC_ASCII_VECTOR("<error>"));