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/javascript/styles/basics.scss

@@ -70,9 +70,8 @@ body {
 
     .dialog img {
       display: block;
-      margin: 20px auto;
-      margin-top: 50px;
-      max-width: 600px;
+      margin: 0 auto;
+      max-width: 470px;
       width: 100%;
       height: auto;
     }