Remove error handling from custom-server-koa's server example (#3982)

Koa's `listeningListener` callback is invoked with no arguments according to the Koa's type definitions, hence error handling is extraneous.

examples/custom-server-koa/server.js

@@ -33,8 +33,7 @@ app.prepare()
   })
 
   server.use(router.routes())
-  server.listen(port, (err) => {
-    if (err) throw err
+  server.listen(port, () => {
     console.log(`> Ready on http://localhost:${port}`)
   })
 })