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/Mobs/PathFinder.cpp

@@ -152,14 +152,8 @@ ePathFinderStatus cPathFinder::GetNextWayPoint(cChunk & a_Chunk, const Vector3d
 				return ePathFinderStatus::PATH_FOUND;
 			}
 		}
-		#ifndef __clang__
-		default:
-		{
-			return ePathFinderStatus::PATH_FOUND;
-			// Fixes GCC warning: "control reaches end of non-void function".
-		}
-		#endif
 	}
+	UNREACHABLE("Unsupported path finder status");
 }