Print a useful error message.

script/start

@@ -28,6 +28,11 @@ server.listen(port, 'localhost', err => {
   console.log(`Server running at http://localhost:${port}`)
 
   const runningApp = run()
+  if (!runningApp) {
+    console.error("Couldn't launch the app. You probably need to build it first. Run `npm run build:dev`.")
+    process.exit(1)
+  }
+
   runningApp.on('close', () => {
     process.exit(0)
   })