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/BlockID.cpp

@@ -388,12 +388,7 @@ AString DamageTypeToString(eDamageType a_DamageType)
 		case dtSuffocating:     return "dtSuffocation";
 		case dtExplosion:       return "dtExplosion";
 	}
-
-	// Unknown damage type:
-	ASSERT(!"Unknown DamageType");
-	#ifndef __clang__
-		return Printf("dtUnknown_%d", static_cast<int>(a_DamageType));
-	#endif
+	UNREACHABLE("Unsupported damage type");
 }