I think it would be good to cancel the print if G29 or G32 fails, since they're important for some setups. Maybe any code that involves homing shouldn't be allowed to continue printing if it fails, but I'm not sure if that would be good for everyone. If G29 and G32 cancelled an SD print upon failure, i think it would be a good stopgap before conditional gcode comes.
I like the idea of a parameter to change the behavior too, but conditional gcode is probably a better solution anyway and if it's a lot of work it probably won't be worth it.
It could be that something could also be done to retry G32 in a different way after a failure of a certain type. For instance if the probe was already triggered on one corner, then move that side up by a couple mm and retry, but that could be done with conditional gcode probably. Also it might be a bit dangerous to do that since the machine could possibly rip itself apart, but w/e.