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.
```

Start at github-api/dist/components/Requestable.js:238 and reproduce the request for a deleted gist. Inspect how the missing response is handled, then verify that the request reports an appropriate error instead of producing the shown TypeError and unhandled rejection.

索引モデルが issue の本文から書いたものです。