extStop should return error if OffActions is empty

Currently if ```extStop``` finds that there are no off actions there is no logging and nil returned. However, extStops sole purpose is to stop the hardware. If this has been called and there are no off actions, then it seems erroneous to return nil as if there are no problems, when in fact stop couldn't happen. We should therefore return an error in this circumstance, and if we don't care we can just ignore this particular error.