Add a check against multiple `GameData` on the rig.

[Lisias]

Vortex is getting traction on the users (apparently CurseForge is dropping support for KSP on their installer... damn!), the installer is not smart enough to handle different zip models.

So it's common to have multiple GameData on a rig handled by Vortex now.

Add a check to detect this problem, making user's life easier when the problem happens.