how to install velodyne_msgs for python

edit

I need to read velodyne_msgs in python. When I import velodyne_msgs it says that the package is not found, so I must install it.

I am running on an Ubuntu 14.04 machine with ROS Indigo installed.

I tried this to install velodyne_msgs:

roslocate info --distro=indigo velodyne_msgs > /tmp/velodyne_msgs.rosinstall
rosinstall ~/self-driving-car/catkin_ws/ /tmp/velodyne_msgs.rosinstall

Here ~/self-driving-car/catkin_ws/ is the one and only catkin workspace on my machine.

When I ran the above commands, it gave me a rosinstall error:

rosinstall operating on /home/ubuntu/self-driving-car/catkin_ws from specifications in rosinstall files  /tmp/velodyne_msgs.rosinstall
(Over-)Writing /home/ubuntu/self-driving-car/catkin_ws/.rosinstall
[common_msgs] Updating /home/ubuntu/self-driving-car/catkin_ws/common_msgs
[common_msgs] Done.
[velodyne_msgs] Fetching https://github.com/ros-drivers/velodyne.git (version master) to /home/ubuntu/self-driving-car/catkin_ws/velodyne_msgs
Cloning into '/home/ubuntu/self-driving-car/catkin_ws/velodyne_msgs'...
remote: Counting objects: 2983, done.
remote: Total 2983 (delta 0), reused 0 (delta 0), pack-reused 2983
Receiving objects: 100% (2983/2983), 581.33 KiB | 0 bytes/s, done.
Resolving deltas: 100% (1664/1664), done.
Checking connectivity... done.
[velodyne_msgs] Done.
(Over-)Writing setup.sh, setup.bash, and setup.zsh in /home/ubuntu/self-driving-car/catkin_ws
ERROR in rosinstall: 
No 'ros' stack detected in candidates [].
Please add the location of a ros distribution to this command.

How do I fix this?