ResourceLoader: Fix sync issues with error reporting

This is about not letting the resource format loader set the error code directly on the task anymore. Instead, it's stored locally and assigned only when it is right to do so.

Otherwise, other tasks may see an error code in the current one before it's state having transitioned to errored. While this, besides the technically true data race, may not be a problem in practice, it causes surprising situations during debugging as it breaks assumptions.

core/io/resource_loader.cpp

@@ -319,7 +319,8 @@ void ResourceLoader::_thread_load_function(void *p_userdata) {
 	}
 	// --
 
-	Ref<Resource> res = _load(load_task.remapped_path, load_task.remapped_path != load_task.local_path ? load_task.local_path : String(), load_task.type_hint, load_task.cache_mode, &load_task.error, load_task.use_sub_threads, &load_task.progress);
+	Error load_err = OK;
+	Ref<Resource> res = _load(load_task.remapped_path, load_task.remapped_path != load_task.local_path ? load_task.local_path : String(), load_task.type_hint, load_task.cache_mode, &load_err, load_task.use_sub_threads, &load_task.progress);
 	if (MessageQueue::get_singleton() != MessageQueue::get_main_singleton()) {
 		MessageQueue::get_singleton()->flush();
 	}
@@ -328,7 +329,8 @@ void ResourceLoader::_thread_load_function(void *p_userdata) {
 
 	load_task.resource = res;
 
-	load_task.progress = 1.0; //it was fully loaded at this point, so force progress to 1.0
+	load_task.progress = 1.0; // It was fully loaded at this point, so force progress to 1.0.
+	load_task.error = load_err;
 	if (load_task.error != OK) {
 		load_task.status = THREAD_LOAD_FAILED;
 	} else {