New error page graphic. Other error page improvements (#5099)

- 500.html generated with admin-set default locale if set
- Error page `<title>` includes Mastodon site title
- 500 title changed to "This page is not
  correct" (ref: <https://www.youtube.com/watch?v=2VCAP_seh1A>)
- 500 content appended with "on our end" to make clear it's
  not user's fault

lib/tasks/assets.rake

@@ -3,15 +3,16 @@
 namespace :assets do
   desc 'Generate 500.html'
   task :generate_500 do
-    html = ApplicationController.render('errors/500', layout: 'error')
-    File.write(Rails.root.join('public', '500.html'), html)
+    I18n.with_locale(ENV['DEFAULT_LOCALE'] || I18n.default_locale) do
+      html = ApplicationController.render('errors/500', layout: 'error')
+      File.write(Rails.root.join('public', '500.html'), html)
+    end
   end
 end
 
 if Rake::Task.task_defined?('assets:precompile')
   Rake::Task['assets:precompile'].enhance do
     Webpacker::Manifest.load
-
     Rake::Task['assets:generate_500'].invoke
   end
 end