extensions: show traceback on load failure if --traceback flag is set
Before this patch, there was no handy way to investigate the reason why
extension couldn't be loaded.
If ui.debug is set, tracebacks of both "hgext.foo" and "foo" are displayed
because the first ImportError could occur at very deep dependency module.
to build a new macosx binary package:
install macpython from http://www.python.org/download/mac
install py2app from http://pythonmac.org/packages/
make sure /usr/local/bin is in your path
run bdist_mpkg in top-level hg directory
find packaged stuff in dist directory