message: Improve warning message when calling get_system() in a multi-systems setup
diff: @@ -276,7 +276,8 @@ System& MavsdkImpl::get_system()
  }
  if (_systems.size() > 1) {
  - LogErr() << "More than one system found:";
  + LogWarn()
  + << "More than one system found. You should be using `get_system(uuid)` instead of `get_system()`!";
  // Just return first system instead of failing.
  return *_systems.begin()->second;
  