Ah, that's great!
Hm, when I would be tasked with wording them, I would imagine, how it would be situated in meatspace.
Like, you couldn't suffessfully submit something. I imagine a waiter getting back to you, apologising and informing you, that you order couldn't be fulfilled (something was missing - what exactly? your dish is out etc). Basically, avoid jargon, help the audience to understand what they can do to recover from an issue.
Forms are the basic kind of interaction between someone entering information and the service responding to it. That's why you will most likely see error messages in this context.