Canonical checked failures returned by application-owned handlers and typed
outbound ports on either endpoint side.
Every payload is a host-authored, already-sanitized string meant for the
client to display: the adapter forwards it verbatim onto the JSON-RPC error
(message plus data), so a user can read — and share with a developer —
what actually happened. Handlers must not put secrets, raw payloads, or
stack traces in these strings.