Marking read, starring, tagging, emptying a folder and cancelling a
scheduled send only checked that the request came back. A `/set` the
server refused - `notUpdated`, `notDestroyed` or a method-level
`error` - resolved as if it had worked: the list showed the new state,
and a cancel the server rejected looked cancelled while the message
still went out.
These writes now go through one check that throws on any of those, so
the store keeps the old state and shows the server's reason.
Linus Rath