closes #125 better error stack

For some reason wrapping `gen.throw()` in a `try/catch` always ends up being catched.
That's why we can't call next(err).

Personally I think we should drastically change the implementation of this, because the stack trace of any error bubbling up in the *yieldabe* should point to the yield line with gen.throw.

Also after a `gen.throw()` you can't yield anymore, the generator appears to be **closed**

Some of this behavior is documented here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Iterators_and_Generators