What happens when a peer closes a channel early?

You can give them a thumbs down rating. No other recourse is possible at this time. But I'm exploring what else we can do in the future when the protocol allows for it.

some of my peers too

maybe there needs to be an early exit penalty.

Yes. I will come with something. The difficulty is that in some cases it’s a technical problem like Tor attacks recently. 

is it fair to say when it's a mutual closure, the one who closes the channel early should be liable?