olderrorhandling.m

- (BOOL)canPurchaseItem:(Item *)item forUser:(User *)user withError:(NSError **)error {
    if ([self.items indexOfObject:item] == NSNotFound) {
        error = [NSError errorWithDomain:StoreErrorDomain
                                    code:PurchaseOutOfStockItemCode
                                userInfo:@{ NSLocalizedDescriptionKey: @"Item is out of stock" }];
        return NO;
    }
    
    if (user.money - item.price < 0) {
        error = [NSError errorWithDomain:StoreErrorDomain
                                    code:PurchaseOutOfStockItemCode
                                userInfo:@{ NSLocalizedDescriptionKey: @"Insufficient fund to purchase this item" }];
        return NO;
    }
    
    [self purchaseItem:item forUser:user];
    return YES;
}

NSError *error = nil;
[self canPurchaseItem:someItem forUser:someUser withError:&error];

if (error != nil) {
    // do something with error occur
} else {

}