message: mission: warning for conflicting item settings
  It doesn't make much sense to have a fly-through mission item but then
  set a loiter time to wait at the waypoint. Therefore, we have now a
  warning message for this case.
diff: @@ -626,6 +626,10 @@ void MissionImpl::assemble_mavlink_messages()
  item_i});
  _mission_data.mavlink_mission_item_messages.push_back(message_delay);
  }
  +
  + if (mission_item_impl.get_fly_through()) {
  + LogWarn() << "Conflicting options set: fly_through=true and loiter_time>0.";
  + }
  }
  if (mission_item_impl.get_camera_action() != MissionItem::CameraAction::NONE) {
  