fix: don't fail when reading a repo that doesn't exist

[marcoieni]

No description provided.

[Kobzol]

I will mark newly merged sync-team PRs with the backport label, so that we know that we should later backport them to team.

[marcoieni]

this is the actual response from github when the repository doesn't exist

{
  "data": {
    "repository": null
  },
  "errors": [
    {
      "type": "NOT_FOUND",
      "path": [
        "repository"
      ],
      "locations": [
        {
          "line": 2,
          "column": 17
        }
      ],
      "message": "Could not resolve to a Repository with the name 'rust-lang/aaa'."
    }
  ]
}

[Kobzol]

I guess that we could detect either null repository or the NOT_FOUND error type (that might be more robust). Or we can do a REST query first to find our if the repo exists or not xD

[Kobzol]

Looks good, thanks!