Добавил страницу 404

src/pages/404.pug

+extends ../pug/layout.pug
+
+block meta
+  title Error 404
+    meta(name='description', content='')
+
+//- block append head
+//-   link(rel='stylesheet', href='css/some.css')
+
+block nav
+  +main-nav('-1')
+
+block content
+  h1 Error 404
+  p Page not found.
+
+//- block footer
+//-   p footer
+
+//- block append page-bottom
+//-   script(src='js/some.js')