Closed generator returns a completed object instead of throwing a error

From ES6 rev20 draft, closed generator returns completed object (the
value is `undefined` and done is `true`).
Since a error thrown in generator is propagated to the caller without
setting status of a thrown generator to "completed", once a generator is
suspended by a error, status becomes "executing" forever. This is filed
as v8:3096

LOG=N
BUG=v8:3097
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/136003003

Patch from Yusuke Suzuki <yusukesuzuki@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18591 ce2b1a6d-e550-0410-aec6-3dcde31c8c00