Promise 中會吞掉(隱藏)throw 例外的錯誤輸出，改用catch來作錯誤處理

 https://eyesofkids.gitbooks.io/javascript-start-es6-promise/content/contents/ch5_flow_n_error.html

// 用throw語句取代reject
const p1 = new Promise((resolve, reject) => {
  throw new Error('rejected!') // 用throw語句

  //相當於用以下的語句
  //reject(new Error('rejected!'))
})

p1.then(val => {
  console.log(val)
  return val + 2
})
  .then(val => console.log(val))
  .catch(err => console.log('error:', err.message))
  .then(val => console.log('done'))

//最後結果:
//error: rejected!
//done