VS: Improve error message when MSBuild is not found
Internally when looking for the Visual Studio instance to use, the Visual Studio generators should ensure that the toolset they need is present in that Visual Studio instance. If it isn't available in any installed Visual Studio instance, then it should produce an error. For example, when running the Visual Studio 2017 generator it needs to ensure that the Visual Studio instance it uses has the v141 toolset installed before using it. Without this logic the generators are sometimes running into issues in the side-by-side case (ex: both Visual Studio 2017 and Visual Studio 2019 are installed).