Ask HN: Failing startup, how to leave?

I am in a complex situation and would like to hear your opinion on how to proceed. I am a cofounder (4 friends from college) of a startup, at first we had great progress with the development of the product (i am one of the hardware developer). We started almost 13 month ago, but till this day we don't have a clear understanding who our customer are or how large the market we target is. When I point out one of these open points(there are many more), I am referred to some google research, which is supposedly completely bulletproof. I think my co-founders are blinded by all those unicorns out there, they think everything is going great and there are only little problems on the way before they get rich. I would like to leave, but I don't want to be the one getting blamed, when the company finally goes bust. How can I handle the situation?