Driver should publish a state that notifies if there is a problem

[toliver]

When the connection is lost, the driver should inform of that through its state topic.

A connection loss might mean a protective stop was triggered in the robot. This circumstance should be notified to any higher level nodes that might be interested.

E.g. when we implement #7 and #2, if we don't notify the higher level nodes that the robot was offline for a certain time, those nodes could keep sending targets to the robot regardless of its state. So when the robot comes back online that could have undesired effects.