Several texture-related fixes.
- Fixed default texture pack being saved as "null".
- Fixed being unable to set sideBlock/edgeBlock/sideLevel if "canServerChangeTextures" is false.
- Fixed textures not getting freed when texture pack is changed.
- Texture pack selection GUI now shows "game broke" message with exception details if default texture failed to load, instead of suppressing errors and leaving the game in an inconsistent state.