refactor(Message): make thread a getter for accuracy (#6215)

* refactor(Message): make thread a getter for accuracy

* docs: reword info block

src/errors/Messages.js

@@ -98,6 +98,7 @@ const Messages = {
   INVALID_ELEMENT: (type, name, elem) => `Supplied ${type} ${name} includes an invalid element: ${elem}`,
 
   MESSAGE_THREAD_PARENT: 'The message was not sent in a guild text or news channel',
+  MESSAGE_EXISTING_THREAD: 'The message already has a thread',
 
   WEBHOOK_MESSAGE: 'The message was not sent by a webhook.',
   WEBHOOK_TOKEN_UNAVAILABLE: 'This action requires a webhook token, but none is available.',