JSON-RPC 2.0 standard and application error codes (throw/catch).
Server control flow only — not the wire Error object. Constants (e.g. INTERNAL_ERROR) are RpcErrorCode values; pass them to to_error / to_response / wire Error as `int` (Vala 0.56 types errordomain members as `int` at call sites).
Static methods only — instance methods on the caught error are not supported in Vala 0.56 yet.