add detail on parse error location and cause

This requires https://github.com/bos/attoparsec/issues/71 to have any impact, but it should work as well before with earlier attoparsecs.


(i just stumbled back over this open browser tab, so i tried to replace the case with a >>=, and then i realized i didn't do that because Result and L.Result are two different types. So I think I need another hint.)