Missing RenderingOpenGL2 module in link fails silently

When porting a VTK-dependant project CMakeLists.txt to VTK9 and using FindNeededModules.py, one would get something like :

find_package(VTK
 COMPONENTS
    FiltersSources
    IOImage
    RenderingCore
    # These modules are suggested since they implement an existing module.
    # Uncomment those you need.
    # InteractionStyle  # implements VTK::RenderingCore
    # RenderingFreeType # implements VTK::RenderingCore
    # RenderingOpenGL2  # implements VTK::RenderingCore
    # RenderingUI       # implements VTK::RenderingCore
)

It is not that easy to know which modules are actually needed for a beginner VTK user. With the example.tgz, forgetting to enable InteractionStyle would create a warning at runtime when going through the related codepath, which is not perfect (a build failure would be better) but understandable.

Forgetting to enable RenderingOpenGL2 would simply prevent the software to open any window without any explanation, warnings or errors at any time.

This should be fixed imo.

FYI: @ben.boeckel @tjcorona