refac(net, exchange, dht) network service errors

Let errors rise up up and away.

@jbenet @whyrusleeping 
These changes are based on this discussion:

https://github.com/jbenet/go-ipfs/pull/69#issuecomment-56246382

It's that it's weird to send errors back down to the service. What does it mean to return error in a handler function? What is the service meant to do with this error?

It would appear that the better thing to do is for the handler to log and perform error handling internally.