Update VMR build scripting to suggest correct branches to use when building

[mmitche]

Update the VMR build scripting to improve UX. For instance:

• If a non-source-only build is attempted from a source-only branch (e.g. look for some file that wouldn't exist in the source-only branch), print a helpful message to the user.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[ViktorHofer]

As mentioned offline, I don't think we should be going that route. I think it's important to make it possible to source-build from the "main" branch.

[mmitche]

Going with a different solution in #4088