Revert "Don't try to show exceptions in a message box if SDL was not initialized"

According to SDL docs, "This function may be called at any time, even before SDL_Init()".

Also fixes an issue where message boxes weren't working due to SDL_Quit already having been called by ~Engine.

This reverts commit 39eea24dc3866760cc40b79b6d57ebbc6799fc73.

Conflicts:
	apps/openmw/main.cpp