I think what the person you're replying to is trying to say is that the web-accessible error message (i.e., the one that JavaScript running in the sending page can read) is intentionally opaque and somewhat misleading, because a more helpful error message would leak information about the response that the sending origin isn't supposed to have. There's typically a more helpful error message in the dev tools (which JavaScript running in a page can't access), but you have to know where to find it.