D3445: dispatch: shore up exit handling

[Yuya Nishihara]

IIRC, SystemExit is caught at callcatch and translated to a status code.

> +    # The logic here attempts to mimic how CPython's pythonrun.c does things.
> +    # Essentially:
> +    #
> +    # * Exit is controlled by returning a value or raising SystemExit.
> +    # * An integer value exits with that exit code.
> +    # * A value of ``None`` is interpreted as ``0``.
> +    # * A non-integer, non-None value results in that value being printed
> +    #   and an exit code of ``1``.
> +    # * SystemExit can have a ``code`` attribute containing the exit value.
>      try:
> -        status = (dispatch(req) or 0)
> +        code = dispatch(req) or 0
>      except error.StdioError as e:
>          err = e
> -        status = -1
> +        code = -1
> +    except SystemExit as err:
> +        try:
> +            code = err.code
> +            err = None
> +        except AttributeError: