[Python-Dev] Handle errors in cleanup code

[Stefan Ring]

> Yury in the comment for PR 2108 [1] suggested more complicated code:
>
>     do_something()
>     try:
>         do_something_other()
>     except BaseException as ex:
>         try:
>             undo_something()
>         finally:
>             raise ex

And this is still bad, because it loses the back trace. The way we do it is:

    do_something()
    try:
        do_something_other()
    except BaseException as ex:
        tb = sys.exc_info()[2]
        try:
            undo_something()
        finally:
            raise ex, None, tb