Different error messages for related resource save fails

[eltiare]

I have this setting enabled: DataMapper::Model.raise_on_save_failure = true

Spent several hours on a problem today whereby a resource wasn't saving and I couldn't figure out why. Turns out a parent resource was have some validation troubles (thanks to a custom validation gone bad) but the only error I got was "DataMapper was unable to save the resource" or some jazz like that. I think it would be supremely useful if a validation save aborts due to a related resource validation failure that a different error be thrown which lists the offending resource. Could save other developers a ton of time and also hair from being pulled out.