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/HTTP/NameValueParser.cpp

@@ -404,10 +404,7 @@ bool cNameValueParser::Finish(void)
 			return true;
 		}
 	}
-	ASSERT(!"Unhandled parser state!");
-	#ifndef __clang__
-		return false;
-	#endif
+	UNREACHABLE("Unsupported name value parser state");
 }