Add support for old-style exception objects.

[rhansen]

Python 2 permits raising old-style objects (objects that aren't instances of the object class). Because these objects are not instances of BaseException, clauses like except BaseException: don't catch them.

Add support for handling old-style exception objects by:

• using a class-less except: clause to catch old-style objects raised by tasks and done callbacks
• using the proper exception object class type when raising a stored old-type exception during result()

[rhansen]

This is sort-of blocking python/typeshed#946.

[agronholm]

Thanks!