Handle errors in OAuth endpoint

[frangio]

Fixes #26.

I've added a test and a fix. Let me know if you had something else in mind.

Jest is reporting missing branch coverage because I'm not testing the default value in case the response does not include an error_description. I introduced the default value just in case, but the documentation I could find seems to always include error_description, so maybe it's not necessary.

[gr2m]

Instead of throwing a custom error, I'd simulate the behavior that would occur if the server responded with a 400 status code. I've created https://github.com/octokit/request-error.js for cases such as this.

The code would look sth like this

throw new RequestError(`${response.data.error_description} (${response.data.error})`, 400, {
  headers: response.headers,
  request: request.endpoint(route, parameters)
});

That way the error can be handled just like other errors such as server outages or request timeouts

[frangio]

I implemented this change but used response.status (which will be 200) rather than 400... Because it doesn't sound like a good idea to fake the status code like that. Let me know if you disagree.

Edit: Reading more carefully I see that you had explicitly suggested emulating a 400 status code. I don't have enough context to understand what is or isn't a good idea here. I'll just do whatever you tell me.

[frangio]

There is a type error that should be fixed by octokit/request-error.js#17.

[gr2m]

I've rebased on latest master and bumped @octokit/request-error to the latest version. Once everything is green I'll go ahead and merge

[gr2m]

Could you do me a favor and apply the same fix to @octokit/auth-oauth-app.js? The same error was reported over there, too, and the very same fix is required: octokit/auth-oauth-app.js#16

[octokitbot]

🎉 This PR is included in version 2.4.1 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

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