Error types are now collected into a single place. Similar errors are
merged.
Request invariant violations are now all under 400 with type
"invalid_request" if it's a client mistake; and if it's caused by a
server restrction, under 403 with type "disabled".
This decouples SQLs from handler logic, makes it easier for auditing and
caching. It also enables the possibility to switch or support multiple
database backends.
- `Msg` or `msg` is now the canonical term for the substructure in a
room. It includes a `chat` subtype and (in the future) other
administration subtypes like member joining or leaving.
- `Message` or `message` can used in human oriented context like docs
and comments, but only when it is unambiguous.
- `message` is not chosen in code because it's hard to type (at least
for me!), and have ambiguous meaning of:
- "Human readable text" in context of `ApiError`'s field.
- "A unit of data transfer, datagram" in context of WebSocket Message.
- `item` is not chosen because it is overly generic.
