PromiseCource(x) should return a rejected promise when accessing x.then
leads to an error.

BUG=347095
LOG=Y
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/194483002

Patch from Yutaka Hirano <yhirano@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19928 ce2b1a6d-e550-0410-aec6-3dcde31c8c00