add error check right when reload nginx fail

[stormltf]

https://github.com/kubernetes/ingress/blob/master/controllers/nginx/pkg/cmd/controller/nginx.go

539 o, e := exec.Command(n.binary, "-s", "reload", "-c", cfgPath).CombinedOutput()
540 if e != nil {
541 return fmt.Errorf("%v\n%v", e, string(o))
542 }