use property observer to send failure event to passthrough delegate
use property observer to send failure event to passthrough delegate

This fixes an issue where a responseError() handler had to be defined in order for the passthrough to receive the failure event. Now the passthrough will always receive the failure event even when no error handlers are set.

Electrode-iOS/ELWebService,TheHolyGrail/Swallow,Electrode-iOS/ELWebService,dwsjoquist/ELWebService