SDL resolution list fixes in launcher graphics page

• Fix error message stating the wrong function failing.
• Display supported resolutions on single-monitor machines.
• Get list of supported resolutions while SDL context is still alive. Probably resolves #5558 (closed) and other SDL_GetNumDisplayModes failed: Video subsystem has not been initialized. errors.

I'm a little concerned that people reporting that error message are saying OpenMW is the thing that doesn't work, but as I can only find that error message in the launcher, I'm assuming they actually mean that the launcher doesn't work.