Deprecated message property for exceptions is used

[meliache]

In b2luigi.core.utils.map_folder an exception is re-raised with a modified exception message, where the original message is appended viaex.message.

    except AttributeError as ex:
        raise type(ex)(
            "Could not determine the current script location. "
            "If you are running in an interactive shell (such as jupyter notebook) "
            "make sure to only provide absolute paths in your settings.\nMore Info:\n" +
            ex.message
        ).with_traceback(sys.exc_info()[2])

I get an error in python 3.9 that the exception has no message property, but this seems to be already deprecated since python 2.6 (see PEP-352).

The obvious solution would be to just use the str(ex) or a format string like f"More info: {ex}". But before fixing this in a PR, my questions is what's the "best" way to re-raise an exception with a modified message in python 3. The raise type(ex)(...).with_traceback(...) construct seems a bit cumbersome to me, is it really necessary? I admit I don't fully understand it, maybe @FelixMetzner who introduced this code in #34 (thanks for that) can help.

[nils-braun]

If you raise an exception within handling another exception, both stack traces and exception messages will be shown (... while handling ...).
You can also choose what is shown with something like raise Ex1 from Ex2, but that should not be necessary here (as the outer exception is the default)

[meliache]

If you raise an exception within handling another exception, both stack traces and exception messages will be shown (... while handling ...).

I saw this when the bug occured, then I got another exception that ex.message doesn't exist 😉

I'll then go forward with the suggested fix.