'Emit a usage message, then exit.'

def _usage_exit(self, argv):

    progname = os.path.basename(argv[0])
    usage_exit_code = 2
    action_usage = '|'.join(self.action_funcs.keys())
    message = ('usage:    %(progname)s    %(action_usage)s' % vars())
    emit_message(message)
    sys.exit(usage_exit_code)
