'``BuildScript.main`` prints a usage error to ``stderr`` if there are
missing command line options.'

def test_usage_error_message(self):

    fake_sys_module = FakeSysModule(argv=[])
    script = BuildScript(sys_module=fake_sys_module)
    try:
        script.main(top_level=FLOCKER_PATH)
    except SystemExit:
        pass
    self.assertEqual('Wrong    number    of    arguments.', fake_sys_module.stderr.getvalue().splitlines()[(-1)])
