message: ADD a test if the buildDir exists (and has an iframe.html) and throw if missing
diff: import fetch from 'node-fetch';
  +import fs from 'fs-extra';
  +import path from 'path';
  import denodeify from 'denodeify';
  import { confirm } from 'node-ask';
  import setupDebug from 'debug';
  @@ -103,7 +105,7 @@ async function prepareAppOrBuild({
  await new Promise((res, rej) => {
  child.on('error', rej);
  child.on('close', code => {
  - if (code > 0) {
  + if (code !== 0) {
  rej(new Error(`${buildScriptName} script exited with code ${code}`));
  }
  res();
  @@ -111,6 +113,16 @@ async function prepareAppOrBuild({
  });
  }
  + const exists = await fs.pathExists(path.join(buildDirName, 'iframe.html'));
  +
  + if (!exists) {
  + if (buildScriptName) {
  + throw new Error(`Storybook was not build succesfully, there are likely errors above`);
  + } else {
  + throw new Error(`The folder you specified (${buildDirName}) doesn't exist`);
  + }
  + }
  +
  log.info(dedent`Uploading your built Storybook...`);
  const isolatorUrl = await uploadToS3(buildDirName, client);
  debug(`uploading to s3, got ${isolatorUrl}`);
  