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/World.cpp

@@ -492,11 +492,7 @@ int cWorld::GetDefaultWeatherInterval(eWeather a_Weather)
 			return Random.RandInt(m_MinThunderStormTicks, m_MaxThunderStormTicks);
 		}
 	}
-
-	#ifndef __clang__
-		ASSERT(!"Unknown weather");
-		return -1;
-	#endif
+	UNREACHABLE("Unsupported weather");
 }
 
 
@@ -851,19 +847,17 @@ eWeather cWorld::ChooseNewWeather()
 	switch (m_Weather)
 	{
 		case eWeather_Sunny:
-		case eWeather_ThunderStorm: return eWeather_Rain;
-
+		case eWeather_ThunderStorm:
+		{
+			return eWeather_Rain;
+		}
 		case eWeather_Rain:
 		{
 			// 1 / 8 chance of turning into a thunderstorm
 			return GetRandomProvider().RandBool(0.125) ? eWeather_ThunderStorm : eWeather_Sunny;
 		}
 	}
-
-	#ifndef __clang__
-		ASSERT(!"Unknown weather");
-		return eWeather_Sunny;
-	#endif
+	UNREACHABLE("Unsupported weather");
 }