Wrap ActorConfiguration.from_file() in a try...except FileNotFoundError
block in robot/helper_actor_config.py. When the config file path does not
exist, print a user-friendly error message to sys.stderr and exit with
code 1 instead of crashing with an unhandled exception and raw traceback.
Also adds a Robot Framework test case 'Missing Config File Exits With
Non-Zero Code And Stderr Message' to robot/actor_configuration.robot to
cover the new error path.
ISSUES CLOSED: #2829
freemo
