* Fixes a number of "<function>: not all control paths return a value" warnings on MSVC.
* Introduces the UNREACHABLE global macro and uses it instead of conditionally compiled switch defaults.
* Move cNBTParseErrorCategory from FastNBT.h into FastNBT.cpp to prevent bad calls to message()
This was mostly done automatically and then visually inspected for obvious errors.
All //-style comments should have a 2-space separation from the code, and 1 space after the comment sign.