Fix error handling in parse_option.
Fix error handling in parse_option.

This was causing the parser to ignore the rest of the file after an
unknown option.

Gwendocg/babeldToS,wlanslovenija/babeld,boutier/babeld,sudomesh/babeld,tcatm/babeld,Gwendocg/babeldToS,dtaht/babeld-shortrtt-metrics,jech/babeld,jech/babeld,wlanslovenija/babeld,Drooids/babeld,woniullb/babeld