Lots of work on exception handling, including tests for traceback printing. We finally have some tests for various exception mode printing, via doctests that exercise all three modes! Also changed handling of sys.exit(X) to only print the summary message, as SystemExit is most often a 'handled' exception. It can still be 100% silenced via '%run -e', but now it's much less intrusive. Added a new %tb magic to print the last available traceback with the current xmode. One can then re-print the last traceback with more detail if desired, without having to cause it again.