nil pointer when failed to create mesos executor driver

[pingginp]

If you don't give any argument when start executor, log.Errorf does not terminate process

https://github.com/paypal/dce-go/blob/develop/dce/main.go#L374-L383

	driver, err := exec.NewMesosExecutorDriver(dConfig)
	if err != nil {
		log.Errorf("Unable to create a ExecutorDriver : %v\n", err.Error())
	}

	_, err = driver.Start()