When an error occurs while record creation in Netsuite via Restlet script, instead of suppressing it with try-catch modules we can selectively pass that error to build a custom error, and throwing that error will deliberately make the HTTP code to 400 bad Request.
var missingIdError = error.create({
name: reponseJson.statusCode,
message: reponseJson.error,
notifyOff: true
});
missingIdError.toString = function () { return missingIdError.message };
throw missingIdError;