Unhandled promise rejection

I've gotten this a couple times recently. If anyone wants to tackle, feel free ❤️

```
$ good-first-issue

? Choose a project: debugger.html
(node:89426) UnhandledPromiseRejectionWarning: HttpError: Validation Failed
at /Users/cyren/GitHub/good-first-issue/node_modules/@octokit/request/dist-node/index.js:66:23
at processTicksAndRejections (internal/process/task_queues.js:89:5)
at async search (/Users/cyren/GitHub/good-first-issue/lib/search.js:18:20)
at async Command. (/Users/cyren/GitHub/good-first-issue/bin/good-first-issue.js:45:20)
(node:89426) 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: 1)
(node:89426) [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 with the failing call in lib/search.js:18 and the command entry point in bin/good-first-issue.js:45. Run good-first-issue against debugger.html to reproduce the GitHub validation error. Done means the failure is handled without an unhandled promise rejection and the CLI exits with an understandable result.

Written by the indexing model from the issue text.