ChanMgr: Add more contextual info to error types.
Now each type has a peer. In some cases this peer is just an address, whereas in others (where key is relevant or address isn't yet narrowed down) it's a full OwnedChanTarget.
Now each type has a peer. In some cases this peer is just an address, whereas in others (where key is relevant or address isn't yet narrowed down) it's a full OwnedChanTarget.