Fix message

When following the rosbuild dynamic_reconfigure tutorial on standard groovy:
http://ros.org/wiki/dynamic_reconfigure/Tutorials/HowToWriteYourFirstCfgFile

I get the following make output:

    [rosbuild] Including /opt/ros/groovy/share/roslisp/rosbuild/roslisp.cmake
    [rosbuild] Including /opt/ros/groovy/share/roscpp/rosbuild/roscpp.cmake
    [rosbuild] Including /opt/ros/groovy/share/rospy/rosbuild/rospy.cmake
    MSG: gencfg_cpp on:tutorial.cfg
    ERROR Finding dependencies for /home/kruset/work/groovy/rosbuild/dynamic_tutorials/cfg/tutorial.cfg
    ...

This does not actually represent an error, but just gendeps writing log data to stderr, and the cmake script assuming anything on stderr is an error, prepending "ERROR".

patch fixes cmake to only print ERROR when gendeps returns with non-zero.

Also while I am at it, cleaned up gendeps a bit for python3.