Craft 3 404 and 503 pages in devMode
Just posting this here since I saw an official answer in Github:


  This changed in Beta 7:
  
  UserException reports are now styled like other exceptions when Dev
  Mode is enabled, with the full stack trace shown.
  
  You can see your 404 template by requesting it directly, e.g. http://mysite.dev/404


https://github.com/craftcms/cms/issues/1613#issuecomment-293366095