Add warning when running start without build (vercel#736)

* add warning when running start without build

* run build before start

* Revert "run build before start"

This reverts commit 171b544.

* exit code & dir

* dont update yarn.lock

* use existsSync

* Add some style changes.

bin/next-start

@@ -3,6 +3,7 @@
 import { resolve } from 'path'
 import parseArgs from 'minimist'
 import Server from '../server'
+import { existsSync } from 'fs'
 
 const argv = parseArgs(process.argv.slice(2), {
   alias: {
@@ -38,6 +39,12 @@ if (argv.help) {
 const dir = resolve(argv._[0] || '.')
 
 const srv = new Server({ dir })
+
+if (!existsSync(resolve(dir, '.next'))) {
+  console.error(`> Could not find the '.next' directory! Try building your app with 'next build' before starting the server.`)
+  process.exit(1)
+}
+
 srv.start(argv.port)
 .then(() => {
   if (!process.env.NOW) {
@@ -48,3 +55,4 @@ srv.start(argv.port)
   console.error(err)
   process.exit(1)
 })
+