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/EntityEffect.cpp

@@ -217,11 +217,7 @@ std::unique_ptr<cEntityEffect> cEntityEffect::CreateEntityEffect(cEntityEffect::
 		case cEntityEffect::effWeakness:       return cpp14::make_unique<cEntityEffectWeakness      >(a_Duration, a_Intensity, a_DistanceModifier);
 		case cEntityEffect::effWither:         return cpp14::make_unique<cEntityEffectWither        >(a_Duration, a_Intensity, a_DistanceModifier);
 	}
-
-	ASSERT(!"Unhandled entity effect type!");
-	#ifndef __clang__
-		return {};
-	#endif
+	UNREACHABLE("Unsupported entity effect");
 }