message: Log an error on overflow.
diff: @@ -1258,6 +1258,11 @@ where
  self.main_purse_spending_limit = res;
  Some(self.main_purse_spending_limit)
  } else {
  + error!(
  + limit = %self.main_purse_spending_limit,
  + spent = %amount,
  + "exceeded main purse spending limit"
  + );
  self.main_purse_spending_limit = U512::zero();
  None
  }
  