Fix empty quiz exercise if user submitted during grader downtime

Fixes #1466

Author: Fatima-Tahir

exercise/views.py

@@ -311,6 +311,13 @@ def get_page(self, request: HttpRequest, students: List[UserProfile]) -> Exercis
                     .first()
                 )
                 if submission:
+                    if self.feedback_revealed and not submission.feedback:
+                        # We cannot show feedback (grader service was probably down), so load a blank exercise page
+                        return self.exercise.load(
+                            request,
+                            students,
+                            url_name=self.post_url_name,
+                        )
                     if self.feedback_revealed:
                         page = ExercisePage(self.exercise)
                         page.content = submission.feedback