lib-storage: If mailbox list iteration fails, don't add INBOX

It possibly should have been returned by the list iteration itself and we
could be returning it wrong here.

Also calling this causes imapc to lost the error message in mailbox_list.

src/lib-storage/list/mailbox-list-iter.c

@@ -562,7 +562,7 @@ mailbox_list_ns_iter_try_next(struct mailbox_list_iterate_context *_ctx,
 	bool has_children;
 
 	if (ctx->cur_ns == NULL) {
-		if (!ctx->inbox_listed && ctx->inbox_list) {
+		if (!ctx->inbox_listed && ctx->inbox_list && !_ctx->failed) {
 			/* send delayed INBOX reply */
 			ctx->inbox_listed = TRUE;
 			inbox_set_children_flags(ctx);