Ensured process exits after static export (#4747) (#4749)

Fixes [this issue](https://github.com/zeit/next.js/issues/4747)

I don't know what is the reason why the process does not finish, because it can be reproduced in this repo in many environments (my local mac os and Netlify pipeline).

However, it fixes the problem and it's 100% safe.

bin/next-export

@@ -57,6 +57,10 @@ const options = {
   outdir: argv.outdir ? resolve(argv.outdir) : resolve(dir, 'out')
 }
 
-exportApp(dir, options).catch((err) => {
-  printAndExit(err)
-})
+exportApp(dir, options)
+  .then(() => {
+    printAndExit('Export successful', 0)
+  })
+  .catch((err) => {
+    printAndExit(err)
+  })