MSIX build AzDO task should fail if it doesn't build an msix package

[florelis]

The build task just calls MSBuild with the appropriate parameters to create a package, assuming that the solution already has a Windows Application Packaging project. If the solution has that project, then everything works fine and the package is created. But if it doesn't or if it is not configured properly, then there is nothing that uses the properties in the command line and the package is not created.

This is not being treated as an error, since MSBuild simply succeeds without knowing that it didn't do what we wanted it to do. We should detect that a package was created and fail if it wasn't.