Handle RCL_RET_UNSUPPORTED for get_non_local_sub count from cyclonedds

[jfinken]

This API is not fully implemented in cyclonedds. More information:

• Add ignore-local-endpoints functionality to avoid double delivery ros2/rclcpp#2202 (comment)
• PR to at least be graceful about it

Here, for the get_non_local_subscription_count method

• detect RCL_RET_UNSUPPORTED and return -1.
• Then handle this condition in the top-level API method

[jfinken]

Closing this PR until a later date. The issue here:

• a rosbag-based subscriber node (which requires serialized messages) within the same process as a publisher
• get_non_local_subscriber_count() will (correctly) return 0, thus inter_process_publish_needed will be false.
• However, IPC currently doesn't support serialized message delivery, thus no messages will reach the rosbag (hence needing inter_process_publish_needed to be true).