Improve error reporting for duplicate object template properties.

BUG=
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/359413007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22112 ce2b1a6d-e550-0410-aec6-3dcde31c8c00