zb,book: Address now non_exhaustive
This is a breaking change but:
- Not more than the addition of a new variant to it in a previous commit.
- It should have been non_exhaustive in the first place so we could think of this as a fix.
Due to an influx of spam, we have had to impose restrictions on new accounts. Please see this wiki page for instructions on how to get full permissions. Sorry for the inconvenience.
This is a breaking change but: