[modules] Improve error messages.

test/message/modules-undefined-export2.js

Index: test/message/modules-undefined-export2.js
diff --git a/test/message/unicode-escape-invalid-2.js b/test/message/modules-undefined-export2.js
similarity index 84%
copy from test/message/unicode-escape-invalid-2.js
copy to test/message/modules-undefined-export2.js
index b83665b1972e0bf26fded82d7edd44fffc3cc829..bf8c2a0c94594a2bf8d27767128ea98291f35a44 100644
--- a/test/message/unicode-escape-invalid-2.js
+++ b/test/message/modules-undefined-export2.js
@@ -1,5 +1,7 @@
 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
+//
+// MODULE
 
-print("\u162P");
+export {x as y};