Improve Python scripts localization

Improve Python scripts localization

* all scripts: add --help and --version options manually so they are
localized, and mark 'Bugs' for translation;

* musicxml2ly.py: use gettext from lilylib, uniformize option
descriptions (always start downcase and do not end with a period);

* musicexp.py, musicxml.py: use encoded writing to stderr and save
translation work by creating error and warning functions.