Wire-level error code. Every code is a Lisp-style symbol string,
rendered as the bare token after :code in the response envelope.
The wire surface is open: well-known engine-emitted codes are
associated &str constants on the type, while script-raised codes
(via (error 'symbol "msg")) flow through the same channel without
a sentinel. Clients pattern-match identically on engine and script
codes — adding a new well-known code is a non-breaking change.