improve exercises mode feedback when tests cannot run due to stack overflow / timeout

[cyrus-]

improve exercises mode feedback when tests cannot run due to stack overflow / timeout

[disconcision]

thought about this one a bit in my last refactor. should be reasonably straightforward. essentially you'll want to modify

• ExerciseMode.impl_validation_view, which calls Cell.test_report_footer_view
• ExerciseMode.impl_grading_view

The former currently just gets the test_results, derived from the ModelResult. If you pass through the whole ModelResult, you should be able to inspect that and add a message for any errors similarly to Cell.live_view.exn_view. For the latter, you'll need to set up a little more infra but it should be basically analogous.