On-error type conformance in the Master_API

[KKostya]

• Changed the default return value from 0 to [] in the @apivalidate arguments for the registerSubscriber and registerPublisher methods.
• Similar typing tweak for the lookupService method.

The ROS Master XMLRPC API (http://wiki.ros.org/ROS/Master_API) defines registerSubscriber and registerPublisher return types to be (int, str, [str]). Yet, on an error, instead of a list of strings, it returns an integer.

This stricter typing is needed for our work on OCaml client project in ROS:
https://github.com/AestheticIntegration/imandra-ros/tree/master-api/rosocaml

[dirk-thomas]

@ros-pull-request-builder retest this please

[dirk-thomas]

Thanks for the patch.