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/ProjectileEntity.cpp

@@ -356,10 +356,7 @@ AString cProjectileEntity::GetMCAClassName(void) const
 		case pkFirework:      return "Firework";
 		case pkFishingFloat:  return "";  // Unknown, perhaps MC doesn't save this?
 	}
-	ASSERT(!"Unhandled projectile entity kind!");
-	#ifndef __clang__
-		return "";
-	#endif
+	UNREACHABLE("Unsupported projectile kind");
 }