Don't trigger error 500 but rather 404 for research practicum

If there is no edition offering the experiments workflow, then give a
404 error on the page.

app/controllers/admin/experiment_projects_controller.rb

@@ -80,6 +80,10 @@ module Admin
 
     def filter_params
       current_course_edition = CourseEdition.supporting_experiment_projects.last
+      if current_course_edition.nil?
+        raise ActionController::RoutingError, 'Not Found'
+      end
+
       edition_id = current_course_edition.id
       round_number = current_course_edition.round