*: handle panic and fatal more consistently

1. etcd fatals if there is critical error in the system and operator should
do something for it
2. etcd panics if there happens something unexpected, and it should be
reported to us to debug.

pkg/flags/urls.go

@@ -49,7 +49,7 @@ func (us *URLsValue) String() string {
 func NewURLsValue(init string) *URLsValue {
 	v := &URLsValue{}
 	if err := v.Set(init); err != nil {
-		log.Panicf("error setting URLsValue: %v", err)
+		log.Panicf("new URLsValue should never fail: %v", err)
 	}
 	return v
 }