OK. Yeah, since StudentInputError delivers the message payload directly to the student, you should only use this error for something that's going to help either the student or course staff debug an issue. Such an ambiguous message is bad UX - it causes confusion and frustration, and worst is that when they email the course staff or edX support, no one can really help them debug.

Let me know how you resolve this issue when you push your changes up, and I'll review.