Handle exception pickling

[ericmjonas]

See the comments on PR #47 at the end -- @ooq is still seeing unpickleable exceptions.

[ericmjonas]

@ooq We were referencing an exception that you had seen that couldn't be serialized and yet still had a problem -- do you have a testcase perhaps ? I can't find the original problem.

[ooq]

@ericmjonas Sorry that I could not recall the exact exception I had.
Just curious, do we have a rough list of 1) what exceptions cannot be pickled? 2) what exceptions can be pickled but cannot be unpickled?
And, is Exception the only object that causes pickle/unpickle issues? Just want to make sure we don't have to stress out about exc_traceback. If exc_traceback is not the worry, why are we doing str(exc_traceback) here?

[ericmjonas]

Sometimes exc_traceback contains pieces of the exception that cannot be pickled and I wanted to be 100% sure things propagated correctly. I don't have a list, but we have example exceptions that I test in test_pickle.py where we create objects (and exceptions) that fail on either pickle or unpickle.