GDScript: Make check for exposed classes more consistent

Some places were already checking if classes from ClassDB were exposed,
while others didn't. This makes the check more consistent to avoid
disparities which can lead to crashes.

modules/gdscript/gdscript_parser.cpp

@@ -4677,7 +4677,7 @@ bool GDScriptParser::export_annotations(AnnotationNode *p_annotation, Node *p_ta
 			if (ScriptServer::is_global_class(arg_string)) {
 				native_class = ScriptServer::get_global_class_native_base(arg_string);
 			}
-			if (!ClassDB::class_exists(native_class)) {
+			if (!ClassDB::class_exists(native_class) || !ClassDB::is_class_exposed(native_class)) {
 				push_error(vformat(R"(Invalid argument %d of annotation "@export_node_path": The class "%s" was not found in the global scope.)", i + 1, arg_string), p_annotation->arguments[i]);
 				return false;
 			} else if (!ClassDB::is_parent_class(native_class, SNAME("Node"))) {