InstallVisualStudio.ps1 exits with 0 when another VS Installer is running

If there's another instance of the VS installer running when you launch https://github.com/dotnet/aspnetcore/blob/main/eng/scripts/InstallVisualStudio.ps1, the script will exit with a success code (0) without actually doing anything. This is because the VS Installer exits with 0 when it's launched while there's another instance running (you can't run 2 instances at the same time). So it's not obvious to devs that `InstallVisualStudio.ps1` didn't do anything in this scenario - we should check for this corner case & exit `InstallVisualStudio.ps1` with an error when it happens.