matteocontrini there is no easy or fast way to ensure that the forum stays up when redis goes down. I believe building in a Redis down catch is treating a symptom instead of the underlying problem. Redis should be treated like your webserver, instead of making app based fallbacks when it goes down, just don't let it go down.
I do however digress and realize that a lot of people are running forums as hobbies. The latest version has catches to see if redis is down. Keep on mind though that your forum will be a lot slower while it's down.
Finally, the reason it will throw a 500 error if Redis is disabled first is because your users get thrown errors if you kill redis while they are using it (not realistically possible to make a catch for this) and it's there to let you know, "hey, do this in a different order." I'll add it to the docs.