txn: Let first error win vs cancellation

Basically since we're doing internal async ops which set the cancellable on
failure, we still want the first error to win since it'll be more useful. See
the docs for `g_task_set_check_cancellable()` for more.

src/daemon/rpmostreed-transaction.c

@@ -590,6 +590,10 @@ transaction_handle_start (RPMOSTreeTransaction *transaction,
                          priv->cancellable,
                          transaction_execute_done_cb,
                          NULL);
+      /* Some of the async ops in rpmostree-core.c will cancel,
+       * but we want the first error to take precedence.
+       */
+      g_task_set_check_cancellable (task, FALSE);
       g_task_run_in_thread (task, transaction_execute_thread);
       g_object_unref (task);
     }