set App::$error on 404 so we don't get two 'Page not found.' page bodies.

Zotlabs/Web/Router.php

@@ -152,6 +152,7 @@ class Router {
 				// pretend this is a module so it will initialise the theme
 				\App::$module = '404';
 				\App::$module_loaded = true;
+				\App::$error = true;
 			}
 		}
 	}