* Make sure to not show pages/404 GIP error from _app having GIP

* Add error for getStaticProps in pages/404 too

* Add support for getStaticProps in pages/404

* Update test

* Implement experimental pages/404.js for custom 404 page

* Make sure to show error for getInitialProps in pages/404 in dev mode also

* Update routes-manifest tests for new value

* Make sure page404 is boolean in routes-manifest

* Rename variables for consistency

* Make sure to only use 404 page for 404 error