Make it possible to debug failed hook imports via use of --traceback Prior to this change, if a Python hook module failed to load (e.g. due to an import error or path problem), it was impossible to figure out why the error occurred, because the ImportErrors that got raised were caught but never displayed. If run with --traceback or ui.traceback=True, hg now prints tracebacks of both of the ImportError instances that get raised before it bails.