Submitting registration with more than 255 chars in short text field gives vague error

[Jaakkonen]

Submitting a registration with over 255 characters in a short answer field gives the default vague "registration did not succeed" style popup. Browser logs show a backend call yielding a 500 error with appropriate error message.

The backend should respond with a 400 Bad request for this error and the UI should have an option to see this message (popup to have a down arrow which extends error details).
For this specific case the UI should also have a red hint and highlight around the box when i.e. 245 chars are typed and there's only 10 more left.