Error when requesting deleted gist

```
(node:1653) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'data' of undefined
at MY_PROJECT/node_modules/github-api/dist/components/Requestable.js:238:29
at
at process._tickCallback (internal/process/next_tick.js:182:7)
(node:1653) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 2)
(node:1653) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
```

從 github-api/dist/components/Requestable.js:238 開始，重現對已刪除 gist 的請求。檢查缺少回應時的處理方式，然後驗證該請求會回報適當的錯誤，而不是產生所示的 TypeError 和未處理的拒絕。

由索引模型根據 Issue 內容生成。