Handle student no longer in the course while grading in LMS frontend

Part of:

- https://github.com/hypothesis/product-backlog/issues/1305

One of the most common issues while grading is a student that's no longer part of the course.

That's now handled in the backend: https://github.com/hypothesis/lms/pull/5333

We need to also handle that error in the frontend and display an informative modal to the instructors.