Port upstream's javascript to the error page

app/javascript/flavours/glitch/packs/error.js

@@ -0,0 +1,13 @@
+import ready from 'flavours/glitch/util/ready';
+
+ready(() => {
+  const image = document.querySelector('img');
+
+  image.addEventListener('mouseenter', () => {
+    image.src = '/oops.gif';
+  });
+
+  image.addEventListener('mouseleave', () => {
+    image.src = '/oops.png';
+  });
+});

app/javascript/flavours/glitch/theme.yml

@@ -7,7 +7,7 @@ pack:
     filename: packs/common.js
     stylesheet: true
   embed: packs/public.js
-  error:
+  error: packs/error.js
   home:
     filename: packs/home.js
     preload: