Improve error handling on setIO service

[fmauch]

We parse the IO service contents here: https://github.com/UniversalRobots/Universal_Robots_ROS_Driver/blob/930a6cb74d8d420410e24c49364bcc2af50bb5ec/ur_robot_driver/src/hardware_interface.cpp#L852-L880

The current implementation allows setting IO pins with higher numbers than existing, which will silently do nothing. It would be nice to check for the pin limits and make the service fail on an illegal request.