rosnode) rosnode_ping to raise exception when socket.err occurs. Update rostest.

<b>How is this change verified</b>:
Passes rostest that runs the modified file and methods.
```
$ rostest rosnode rosnode.test
```

<b>Effect to other modules</b>:
According to the command search (see result below), the modified method seems to be used only within the same file, and a pkg that I maintain.
```
/opt/ros/groovy$ grep -r 'rosnode_ping(' .
./lib/python2.7/dist-packages/rosnode/__init__.py:def rosnode_ping(node_name, max_count=None, verbose=False):
./lib/python2.7/dist-packages/rosnode/__init__.py:        if rosnode_ping(node, max_count=1, verbose=verbose):
./lib/python2.7/dist-packages/rosnode/__init__.py:        rosnode_ping(node_name, verbose=True, max_count=options.ping_count)
Binary file ./lib/python2.7/dist-packages/rqt_moveit/moveit_widget.pyc matches
./lib/python2.7/dist-packages/rqt_moveit/moveit_widget.py:            is_node_running = rosnode_ping(nodename, 1)
```

(As a reference to the used version, `ros-groovy-rosnode` is 1.9.44-0quantal-20130326-0855-+0000)