message: core: error message if INT messages not supported
  This will help debugging this issue the next time it happens.
diff: @@ -20,6 +20,7 @@ std::weak_ptr<MAVLinkMissionTransfer::WorkItem> MAVLinkMissionTransfer::upload_i
  {
  if (!_int_messages_supported) {
  if (callback) {
  + LogErr() << "Int messages are not supported.";
  callback(Result::IntMessagesNotSupported);
  }
  return {};
  @@ -38,6 +39,7 @@ MAVLinkMissionTransfer::download_items_async(uint8_t type, ResultAndItemsCallbac
  {
  if (!_int_messages_supported) {
  if (callback) {
  + LogErr() << "Int messages are not supported.";
  callback(Result::IntMessagesNotSupported, {});
  }
  return {};
  @@ -57,6 +59,7 @@ MAVLinkMissionTransfer::receive_incoming_items_async(
  {
  if (!_int_messages_supported) {
  if (callback) {
  + LogErr() << "Int messages are not supported.";
  callback(Result::IntMessagesNotSupported, {});
  }
  return {};
  