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.

Rédigé par le modèle d'indexation à partir du texte de l'issue.