app/config/error folder you will find three files.
error.js you will have the handler for errors 404 and 500.
They come with a default behavior, but you can change it.
The code for Error 500 fill run when a uncaught exception happen in your app, so take care on it! =)
The other files are the views of the pages of error 404 and 500, they are EJS files, but you can change their extension for your favorite template engine you've set.