Forcible isolate termination: odd v8 behaviour

[paddybyers]

This commit:

1f7df3b

addresses an issue if a sub-script (run by runInNewContext()) is forcibly terminated (via node::Isolate::Stop()).

The root cause is this:

Isolate::PropagatePendingExceptionToExternalTryCatch()
...
  } else if (thread_local_top_.pending_exception_ ==
             heap()->termination_exception()) {
    try_catch_handler()->can_continue_ = false;
    try_catch_handler()->exception_ = heap()->null_value();
...

So .. when a TerminationException is delivered to an executing sub-script, the innermost TryCatch (which is in EvalMachine()) knows that the exception can't be caught, but the exception value itself is not copied. So, when that TryCatch is rethrown, the next TryCatch up the stack has no idea that it's a TerminationException, and happily just propagates a normal exception with null value.

I don't understand why v8 thinks this is the right thing to do. However, we can deal with it in node.