rqt_action) Bug fix (following error now doesn't occur)

```
rosuser@rosm:~$ Traceback (most recent call last):
  File "~/catkin_ws/src/rqt_common_plugins/rqt_action/src/rqt_action/msg_widget.py", line 149, in _handle_mouse_press
    self._rightclick_menu(event)
  File "~/catkin_ws/src/rqt_common_plugins/rqt_action/src/rqt_action/msg_widget.py", line 191, in _rightclick_menu
    action == raw_action)
  File "~/catkin_ws/src/rqt_common_plugins/rqt_action/src/rqt_action/rosaction.py", line 703, in get_action_text
    raise ROSActionException("Unable to load action [%s]: %s"%(type_, e))
rqt_action.rosaction.ROSActionException: Unable to load action [std_msgs/Header]: Cannot locate message [Header] in package [std_msgs] with paths [['/opt/ros/groovy/share/std_msgs/action']]
```