What to do on errors; most often called if a message is not found.
"silent": Ignore the error; use the message’s id as the replacement message.
"error": Throw the error.
"warn"(default): Print a warning in the console and use the message’s id as the replacement message.
(error) => any: A custom function that is called with an
path: stringfields set. The return falue is used as the replacement message.
onError?: 'error' | 'silent' | 'warn' | ((error: MessageError) => any);