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

app/views/errors/500.html.haml

@@ -1,5 +1,5 @@
 - content_for :page_title do
-  = t('errors.500')
+  = t('errors.500.title')
 
 - content_for :content do
-  = t('errors.500')
+  = t('errors.500.content')