Add error message when topic can't connect (roscpp and rospy)

[ahendrix]

One of the hard-to-debug problems in ROS is when a publisher is active and registered with the ROS master, but has a bad IP/DNS/ROS_HOSTNAME, so remote clients are not able to connect.

We should consider adding an error message (or making an existing error more visible) to the client libraries whenever the TCP connection to a publisher fails; particularly if it fails during name resolution.

[dirk-thomas]

For rospy you can check if changing the two lines from logdebug() to logwarn() would be what you want:
https://github.com/ros/ros_comm/blob/groovy-devel/clients/rospy/src/rospy/impl/registration.py#L315
https://github.com/ros/ros_comm/blob/groovy-devel/clients/rospy/src/rospy/impl/registration.py#L318

For the cpp part it will be pretty difficult to achieve. Any new connection attempt is scheduled with a PendingConnection. This one tries forever. Adding a warning at that level will be difficult since in a lot of scenarios the connection will intentionally only be established after some time (when the subscriber actually appears).