do not try to open config file if not provided
do not try to open config file if not provided

fix attempt to access res variable not assigned if exception is raised in main.

BlueBrain/NeuroM,eleftherioszisis/NeuroM,liesbethvanherpe/NeuroM,mgeplf/NeuroM,juanchopanza/NeuroM,wizmer/NeuroM,lidakanari/NeuroM