x/build/maintner: GitHub issue becoming 404 (e.g., due to being deleted) is not reflected in model

[dmitshur]

GitHub issue #23772 is currently 404.

If it's possible to do so in the scope of the work that maintner already does (i.e., it doesn't require going out of its way), then it should detect that and set its NotExist field gets to true.

c, err := godata.Get(context.Background())
if err != nil {
	panic(err)
}
i := c.GitHub().Repo("golang", "go").Issue(23772)

fmt.Println(i.NotExist)

// Output: false (but should be true)

/cc @bradfitz @jmdobry

(This came up from #30182.)

[gopherbot]

Change https://golang.org/cl/161906 mentions this issue: cmd/gopherbot: handle 404 GitHub issues in freezeOldIssues task

[gopherbot]

Change https://go.dev/cl/442156 mentions this issue: cmd/gopherbot: skip more deleted issues in addGitHubComment

[gopherbot]

Change https://go.dev/cl/593056 mentions this issue: cmd/gopherbot: handle gone issues in setMilestone too

[melinath]

This is breaking maintner for our use case - a spam PR was deleted, and now maintner fails to sync-with-github with:

404 Not Found [{Resource: Field: Code: Message:Could not resolve to a node with the global id of '<ID>'.}]

We don't care about the deleted PR, but maintner breaks trying to process it and exits with a non-zero code.