Add custom error pages cookbook entry

Author: bbrtj

lib/Thunderhorse/Cookbook.pod

@@ -4,6 +4,33 @@ Thunderhorse::Cookbook - Reusable coding patterns
 
 =head1 RECIPES
 
+=head2 Custom error pages
+
+Making prettier app-wide error pages is easy by overriding
+L<Thunderhorse::App/render_error>:
+
+	async sub render_error ($self, $controller, $ctx, $code, $message = undef)
+	{
+		$controller //= $self->controller;
+		my $html;
+
+		try {
+			$html = $controller->template("error/$code");
+		}
+		catch ($ex) {
+			$html = $controller->template('error/error', {title => status_message($code)});
+		}
+
+		await $ctx->res->status($code)->html($html);
+	}
+
+For example for error 404, code above will try to render C<error/404.tt>
+template. If it is not found, it will render more generic C<error/error.tt>
+instead.
+
+Same can be done on controller level, which means different parts of the system
+can use different error pages without any issue.
+
 =head2 Reusing common bridge across controllers
 
 If you need to make a common bridge that will be used across the system, it is