VS: Improve error message when no VS is installed
CMake queries the VS Installer tool through a COM interface to get a list of VS installations. If none is found then we report a brief error:
Generator Visual Studio 16 2019 could not find any instance of Visual Studio.
This error is sometimes encountered even when VS seems to be installed because we ask the VS Installer only for complete installations. If an installation is unfinished because it requires a reboot then we do not see it. We may be able to improve the error message by performing a second query to the VS Installer tool that asks for all instances. We shouldn't use an unfinished installation, but knowing that it is there may help improve the error message.