LibJS: Rename JS_ENUMERATE_{ERROR_SUBCLASSES => NATIVE_ERRORS}

The fact that they *are* subclasses is an implementation detail and
should not be highlighted. The spec calls these NativeErrors, so let's
use that.
Also added a comment explaining *why* they inherit from Error - I was
about to change that :^)

Userland/Libraries/LibJS/Runtime/Error.cpp

@@ -43,7 +43,7 @@ Error::Error(Object& prototype)
     {                                                                                                                   \
     }
 
-JS_ENUMERATE_ERROR_SUBCLASSES
+JS_ENUMERATE_NATIVE_ERRORS
 #undef __JS_ENUMERATE
 
 }