Deal with covered switches consistently (#4161)

* 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()

src/Generating/StructGen.cpp

@@ -274,6 +274,7 @@ int cStructGenTrees::GetNumTrees(
 				return 0;
 			}
 		}
+		UNREACHABLE("Unsupported biome");
 	};
 
 	int NumTrees = 0;