* refactor `SubscriptionClosed`

I noticed that we "trying" to sending "close messages" on subscriptions
that already has been terminated by the client which is unnecessary.

Moreover, this introduces `fn is_closed` to check whether a given
channel is closed without having to send an actual message on the channel.

* cargo fmt

* Update types/src/error.rs

Co-authored-by: David <[email protected]>

* fix nits

* more grumbles

* make rust stable happy

* SubscriptionClosed: add deny unknown fields

Co-authored-by: David <[email protected]>