* Check for the existence of the script provided
* Pass an absolute path to the service

This enables to directly launch a python file within any roslaunch file
Adapted by F.Keith for the extraction of the messages.

Reimplement runsource to reset the cache in case of SyntaxError

Behaviour of runsource based on:
http://docs.python.org/2/library/code.html

Retry the command (one time only) if the connection was lost