If I am given a task that I know I am going to probably fail, I will say something, but I will still try.

My tasks are geared to push a limit, overstep my comfort zone or find out something new about myself.

He may even be aware that I will fail, but as long as I try and make an effort to do something he asks of me, then I have done much more for him by attempting something than not bothering to experience it at all.

Punishments for me are not something I would choose, as mine are geared to be extremely uncomfortable.
I would expect one if I didn't want to try, but I don't think he would ever use them as a means to push me further. That's pushing limits when I am not ready to be pushed.

Sometimes he does say that there will be a punishment if I do not accomplish what I am asked to do, and while the punishment is not very severe, he uses it when he knows I am digging in my heels and he wants to push a bit further.

I wouldn't worry about failing, as long as you try, and who knows maybe you will be able to go a bit further than you thought you could.