python: Improve progress callbacks.
* lang/python/helpers.c (pyProgressCb): Stash python errors, convert 'what' to Unicode object. * lang/python/pyme/core.py (Context.set_progress_cb): Hand in 'self'. * lang/python/tests/t-callbacks.py: Test progress callbacks.
- Signed-off-by: Justus Winter <justus@gnupg.org>