UX: Use the same 404 page for the `unknown` route and exception handler

app/assets/javascripts/discourse/controllers/exception.js.es6

@@ -32,6 +32,7 @@ export default Ember.Controller.extend({
     if (this.get('thrown.jqTextStatus') === "timeout") return true;
     return false;
   }.property(),
+
   isNotFound: Em.computed.equal('thrown.status', 404),
   isForbidden: Em.computed.equal('thrown.status', 403),
   isServer: Em.computed.gte('thrown.status', 500),

app/assets/javascripts/discourse/routes/app-route-map.js.es6

@@ -1,6 +1,7 @@
 export default function() {
   // Error page
   this.route('exception', { path: '/exception' });
+  this.route('exception-unknown', { path: '/404' });
 
   this.route('about', { path: '/about', resetNamespace: true });
 

app/assets/javascripts/discourse/routes/application.js.es6

@@ -83,6 +83,10 @@ const ApplicationRoute = Discourse.Route.extend(OpenComposer, {
         c.error(xhrOrErr);
       }
 
+      if (xhrOrErr && xhrOrErr.status === 404) {
+        return this.transitionTo('exception-unknown');
+      }
+
       exceptionController.setProperties({ lastTransition: transition, thrown: xhrOrErr });
 
       this.intermediateTransitionTo('exception');

app/assets/javascripts/discourse/routes/exception-unknown.js.es6

@@ -0,0 +1,7 @@
+import UnknownRoute from 'discourse/routes/unknown';
+
+export default UnknownRoute.extend({
+  renderTemplate() {
+    this.render('unknown');
+  }
+});