Cannot create a PR if source branch is not included in remotes settings

[sbatten]

1. Create a typical setup where you have an upstream remote and your fork remote added in your local repo.
2. Rename them. E.g. microsoft and sbatten respectively.
3. Use the githubPullRequests.remotes setting to add just microsoft as that's the only remote I care about PRs for.
4. Try to create a PR where the source branch is from the fork sbatten in my case.

🐛 => this silently fails. Console logs show that it does not believe the fork is a github repo.

[RMacfarlane]

Pushed a change to at least show a notification in this case with an option to configure remotes