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/Entities/ArrowEntity.cpp

@@ -66,10 +66,7 @@ bool cArrowEntity::CanPickup(const cPlayer & a_Player) const
 		case psInSurvivalOrCreative: return (a_Player.IsGameModeSurvival() || a_Player.IsGameModeCreative());
 		case psInCreative:           return a_Player.IsGameModeCreative();
 	}
-	ASSERT(!"Unhandled pickup state");
-	#ifndef __clang__
-		return false;
-	#endif
+	UNREACHABLE("Unsupported arrow pickup state");
 }