Error marker loading failure is not handled

Failure to load a mesh throws an exception. It's normally caught and the mesh is promptly replaced with the error marker... Unless the error marker model itself cannot be loaded, which leads to process termination because that exception is not caught. It makes sense to try and look for a different format substitute and finally, the built-in error marker in this case.

That aside, I think the error marker model should be cached. Right now every single missing or broken model causes the error marker model to be reloaded. This is not normally a problem as the error marker's geometry is simple, but when the error marker doesn't come from Morrowind this results in spam.