Stacktrace missing when tornado.concurrent.Future throws an exception

[nparrish]

I'm using graphene in a Tornado-based web application. Since most of our async operations use Tornado's coroutines, we end up wrapping those Futures in Promises. When the coroutine throws an exception, we usually don't get a stack trace.

I have a fix, but there may be a better way:

https://github.com/syrusakbary/promise/pull/47