Introduce MaybeHandle to police exception checking in handlified code.

R=ishell@chromium.org

Review URL: https://codereview.chromium.org/222163002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20457 ce2b1a6d-e550-0410-aec6-3dcde31c8c00