forge/github: Surface GraphQL API error codes

[abhinav]

shurcooL/githubv4 hides the error codes in the returned errors. There's an open issue to resolve this but it hasn't seen progress in years.

As a workaround, we'll interpret the GraphQL errors ourselves at the HTTP transport layer -- at least for now. In the future, we should switch to a different GraphQL client library.

Closes #437

[abhinav]

This change is part of the following stack:

• 440 ◀

_{Change managed by git-spice.}