Improved guidance on video installation issues

To support (die roll) videos, the game may need to install a codec. It tries to do this automatically on initial startup or when the user runs the menu item Options->Video->Test and in resulting dialogs selects to (Re-)install the video codec.

However, on modern Windows OS's this might not succeed when the application is not run as an Administrator, which it by default is not.

In this patch, the error message on installation failure is expanded to contain a system error message when available (like 'The requested operation requires elevation') and two optional ways to try to workaround the issues: - Copying the installer executable to a different location before closing the dialog, and later trying to run it manually (as an Administrator). - Running the game executable manually (not via Steam) as an Administrator to get through the video codec installation issues, after which the game and the videos should work ok even when run normally via Steam.