You don't actually need the FaceBones NIF to make facial animations. Animations are the .TRI's job. It's more for character creation and stretching the mesh so that it would fit armors better. Your facebones NIF might be corrupted, or made incorrectly. I've attempted to make proper FaceBone NIFs, but only had some of them crash. FaceBones are automatically loaded up into the game, that's why they aren't listed in the .esps.