Attempt at fixing hang in promise.

The specific case that causes an issue is Cont#eval,
that results in Unfulfilled and then later errors out.
This meant the error handling chain was broken,
and Promises waiting on a result would never be
completed (or broken).

Fixes #267.

@runarorama If you had time, could you have a look at this for me. I think what I am doing is somewhat of a kludge, but I have not yet been able to come up with a better formulation for the error handling to ensure it always propagates.