Honestly I've been given tasks that I was absolutely certain was impossible... well wasn't I shocked that I actually pulled it off?! But, for no reason would I not tell Him that I thought the task was impossible. He will usually ask me why I think that. If I have a valid reason for it being impossible, He will alter the task. He doesn't want me to fail. However, often enough if my reason is simply "I don't think I can do it" He will push me to try. Usually if I fail at a task that I honestly tried at, I'm not punished.

To answer the question though, yes you should tell Him if you think a task is impossible. He might think different. I have no idea what the task was so I can't say. But, our lifestyle requires honesty and communication. So you should always be open and honest with your Dom, He could simply be testing your limits, or just unaware of how difficult the task was.