do not render the development environment for exercises without visible files

app/controllers/exercises_controller.rb

@@ -82,6 +82,7 @@ class ExercisesController < ApplicationController
   private :handle_file_uploads
 
   def implement
+    redirect_to(@exercise, alert: t('exercises.implement.no_files')) unless @exercise.files.visible.exists?
     @submission = current_user.submissions.where(exercise_id: @exercise.id).order('created_at DESC').first
     @files = (@submission ? @submission.collect_files : @exercise.files).select(&:visible).sort_by(&:name_with_extension)
     @paths = collect_paths(@files)

app/views/application/_flash.html.slim

@@ -1,3 +1,3 @@
 #flash data-message-failure=t('shared.message_failure')
-  - %w[danger info notice success warning].each do |severity|
-    p.alert.flash class="alert-#{severity == 'notice' ? 'success' : severity}" id="flash-#{severity}" = flash[severity]
+  - %w[alert danger info notice success warning].each do |severity|
+    p.alert.flash class="alert-#{{'alert' => 'warning', 'notice' => 'success'}.fetch(severity, severity)}" id="flash-#{severity}" = flash[severity]