Add middleware in admin and forum to some errors via the frontend

src/Admin/AdminServiceProvider.php

@@ -58,6 +58,7 @@ class AdminServiceProvider extends AbstractServiceProvider
                 HttpMiddleware\SetLocale::class,
                 'flarum.admin.route_resolver',
                 HttpMiddleware\CheckCsrfToken::class,
+                'flarum.http.frontend_handler',
                 Middleware\RequireAdministrateAbility::class
             ];
         });

src/Forum/ForumServiceProvider.php

@@ -69,6 +69,7 @@ class ForumServiceProvider extends AbstractServiceProvider
                 'flarum.forum.route_resolver',
                 HttpMiddleware\CheckCsrfToken::class,
                 HttpMiddleware\ShareErrorsFromSession::class,
+                'flarum.http.frontend_handler',
                 HttpMiddleware\FlarumPromotionHeader::class,
             ];
         });

src/Http/HttpServiceProvider.php

@@ -16,6 +16,10 @@ use Flarum\Settings\SettingsRepositoryInterface;
 use Flarum\User\User;
 use Flarum\User\UsernameSlugDriver;
 use Illuminate\Support\Arr;
+use Flarum\Foundation\ErrorHandling\Registry;
+use Flarum\Foundation\ErrorHandling\Reporter;
+use Flarum\Foundation\ErrorHandling\FrontendFormatter;
+use Flarum\Foundation\ErrorHandling\WhoopsFormatter;
 
 class HttpServiceProvider extends AbstractServiceProvider
 {
@@ -58,9 +62,28 @@ class HttpServiceProvider extends AbstractServiceProvider
 
             return $compiledDrivers;
         });
+
         $this->container->bind(SlugManager::class, function () {
             return new SlugManager($this->container->make('flarum.http.selectedSlugDrivers'));
         });
+
+        $this->container->singleton('flarum.http.frontend_exceptions', function () {
+            return [
+                NotAuthenticatedException::class, // 401
+                PermissionDeniedException::class, // 403
+                ModelNotFoundException::class, // 404
+                RouteNotFoundException::class, // 404
+            ];
+        });
+
+        $this->container->singleton('flarum.http.frontend_handler', function () {
+            return new Middleware\HandleErrors(
+                $this->container->make(Registry::class),
+                $this->container['flarum']->inDebugMode() ? $this->container->make(WhoopsFormatter::class) : $this->container->make(FrontendFormatter::class),
+                $this->container->tagged(Reporter::class),
+                $this->container->make('flarum.http.frontend_exceptions')
+            );
+        });
     }
 
     /**