message: core: add two err statements
  These should not happen but until we are sure they don't, let's watch it
  closer.
diff: @@ -166,7 +166,7 @@ void MavlinkCommandSender::receive_command_ack(mavlink_message_t message)
  auto work = *it;
  if (!work) {
  - LogErr() << "no work!";
  + LogErr() << "No work available! (should not happen #1)";
  return;
  }
  @@ -275,6 +275,11 @@ void MavlinkCommandSender::receive_timeout(const CommandIdentification& identifi
  for (auto it = _work_queue.begin(); it != _work_queue.end(); ++it) {
  auto work = *it;
  + if (!work) {
  + LogErr() << "No work available! (should not happen #2)";
  + return;
  + }
  +
  if (work->identification != identification) {
  continue;
  }
  