Proposal: Sub underflow results in ZeroCurrency instead of noop

Created by: ChrisSchinnerl

There have been multiple reported issues caused by a Currency underflow. I think we have fixed it twice by simply setting the currency to 0. Issue #2863 brought this to my attention once more and this time the consequence of not returning 0 in case of an underflow might be more severe. The host might post too much collateral and later fail its own verification when the renter uploads/changes data. (could be related to the issue of Fornax on discord) By changing the behavior of Sub we should be notified when an underflow happens and can still expect somewhat sane behavior.