Enumerate error conditions and describe semantics

[littledan]

@wycats and @bradleymeck have both raised this issue: what happens in various cases where one side understands the subsetting protocol and the other doesn't, or where both sides understand it, but some intermediary does not? How are errors, where the other side gives invalid messages, handled? We should enumerate all the cases we can think of, and ensure that the details are handled in a useful way.