Fix block page when setting "exercises_hidden_when_no_start_date" is on

In configuration.php

$_custom['exercises_hidden_when_no_start_date']

See BT#15631

main/exercise/exercise_submit.php

@@ -967,6 +967,7 @@ if (isset($_custom['exercises_hidden_when_no_start_date']) &&
         );
         if (!in_array($origin, ['learnpath', 'embeddable'])) {
             Display::display_footer();
+            exit;
         }
     }
 }