Rethrow exception when swallowing it is not intended.

We're trying to make v8::TryCatch nestable.
In preparation for that, this CL adds ReThrow for nested trycatches.

BUG=362388

Review URL: https://codereview.chromium.org/313033002

git-svn-id: svn://svn.chromium.org/blink/trunk@176411 bbb929c8-8fbe-4397-9dbb-9b2b20218538