Assert in the Handle constructor that the object is not a failure.

I have run our own tests in debug mode and the WebKit layout tests in
debug mode and there are no regressions.
Review URL: http://codereview.chromium.org/9114

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@691 ce2b1a6d-e550-0410-aec6-3dcde31c8c00