Recommended improvements to EventScript.py

[jamesthinkbox]

Referring to https://github.com/ThinkboxSoftware/Deadline/tree/master/Custom/events/EventScript

@tokejepsen Recommendations for improvements: After retrieving the script's path from the ExtraInfo, run a path mapping on it to convert it to the Slave's current platform (so that the script works cross-platform). After doing the path mapping, confirm that the script file is reachable from the Slave before calling execfile().