server/app_error_handler.ts

The assembled app's onError — what a throw that escapes every route catch answers.

Route handlers and the RPC dispatcher map their own throws (http/route_spec.ts, actions/perform_action.ts); what reaches here is a throw from middleware — above all the credential legs, whose failed read ends the request rather than reading as anonymous. Two answers:

• a failed pool checkout (DbCheckoutError) is the database unavailable: 503 {"error": "service_unavailable"}, the Rust spine's fuz_http::db_unavailable_response (and its CredentialReadFault of the checkout kind);
• anything else is Hono's default error handler, reproduced byte for byte — an HTTPException's own response, else 500 text/plain; charset=UTF-8 Internal Server Error, the body the Rust CredentialReadFault of the query kind answers with.