Use content for error page

src/Foundation/ErrorHandling/FrontendFormatter.php

@@ -10,6 +10,9 @@
 namespace Flarum\Foundation\ErrorHandling;
 
 use Flarum\Frontend\Controller;
+use Flarum\Http\Content\PermissionDenied;
+use Flarum\Http\Content\NotAuthenticated;
+use Flarum\Http\Content\NotFound;
 use Flarum\Settings\SettingsRepositoryInterface;
 use Illuminate\Contracts\Container\Container;
 use Psr\Http\Message\ResponseInterface as Response;
@@ -55,6 +58,16 @@ class FrontendFormatter implements HttpFormatter
     {
         $frontend = $this->container->make("flarum.frontend.forum");
 
+        if ($error->getStatusCode() === 401) {
+            $frontend->content(new NotAuthenticated);
+        }
+        elseif ($error->getStatusCode() === 403) {
+            $frontend->content(new PermissionDenied);
+        }
+        elseif ($error->getStatusCode() === 404) {
+            $frontend->content(new NotFound);
+        }
+
         return (new Controller($frontend))->handle($request)->withStatus($error->getStatusCode());
     }
 }

src/Http/Content/NotAuthenticated.php

@@ -0,0 +1,24 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Http\Content;
+
+use Flarum\Frontend\Document;
+use Psr\Http\Message\ServerRequestInterface as Request;
+
+class NotAuthenticated
+{
+    public function __invoke(Document $document, Request $request)
+    {
+        $document->title = 'Not Authenticated';
+        $document->payload['errorCode'] = 103;
+
+        return $document;
+    }
+}

src/Http/Content/NotFound.php

@@ -0,0 +1,24 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Http\Content;
+
+use Flarum\Frontend\Document;
+use Psr\Http\Message\ServerRequestInterface as Request;
+
+class NotFound
+{
+    public function __invoke(Document $document, Request $request)
+    {
+        $document->title = 'Not Found';
+        $document->payload['errorCode'] = 404;
+
+        return $document;
+    }
+}

src/Http/Content/PermissionDenied.php

@@ -0,0 +1,24 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Http\Content;
+
+use Flarum\Frontend\Document;
+use Psr\Http\Message\ServerRequestInterface as Request;
+
+class PermissionDenied
+{
+    public function __invoke(Document $document, Request $request)
+    {
+        $document->title = 'Permission Denied';
+        $document->payload['errorCode'] = 403;
+
+        return $document;
+    }
+}