x/build/cmd/gopherbot: handle transferred issues

[toothrot]

Gopherbot currently gets stuck in an error loop when it encounters an issue that has been transferred, such as golang/vscode-go#298, which is now #40101.

2020/08/07 19:14:35 close stale WaitingForInfo: GET https://api.github.com/repos/golang/vscode-go/issues/298/comments?per_page=1000&since=2020-07-07T17%3A07%3A54Z: 404 Not Found []
2020/08/07 19:14:35 gopherbot ran in 3.547726498s
2020/08/07 19:14:35 sleeping 30s after previous error.

It shouldn't get stuck when this happens.

/cc @golang/osp-team

[gopherbot]

Change https://golang.org/cl/247409 mentions this issue: cmd/gopherbot: skip commenting on nonexistent issues

[gopherbot]

Change https://golang.org/cl/256040 mentions this issue: cmd/gopherbot: skip labeling nonexistent issues

[dmitshur]

This issue also affected a recently added "set vscode-go milestones" task. There is more information available in #45461.

[gopherbot]

Change https://golang.org/cl/309069 mentions this issue: cmd/gopherbot: only milestone new vscode-go issues as Untriaged

[gopherbot]

Change https://go.dev/cl/397234 mentions this issue: maintner: mark issues as NotExist when its comments are gone

[gopherbot]

Change https://go.dev/cl/397634 mentions this issue: cmd/gopherbot: remember which issues are discovered to be gone

[gopherbot]

Change https://go.dev/cl/397635 mentions this issue: cmd/gopherbot: skip issues that are gone more consistently