Errorlevel 1 should abandon the backup job silently:
Some time ago, I made a sample script to return an errorlevel depending on one or more discovered network settings (current associated SSID, reachability of an IP address, existence of a shared folder, check if wired ethernet adapter is connected):
The feature request was implemented over a year ago, so a script like this should work using a current Duplicati version (not tested by me!)