Provide a way to return traceback information for failed tasks.

Presently, tracebacks are unavailable for failed tasks (under python2, under python3 I think exceptions have a `traceback` attribute). Provide a method to receive tracebacks.

The current return value for arbiter is of the type: `namedtuple('Result', ['success', 'value'])`
Options for adding this are:
- Have value be `namedtuple('Failure', ['exception', 'traceback'])` in the case of `success=False`
- Have value be a custom exception that contains traceback info and the original exception
- mimicking however Python3 does it
- Change the return value to `namedtuple('Result', ['success', 'value', 'traceback'])`

Start by locating the arbiter entry point and the current Result namedtuple, then trace how failed tasks produce their return value. Compare the proposed failure representations and Python 2 exception behavior; done means callers can reliably receive the original exception and its traceback for failed tasks.

Written by the indexing model from the issue text.