Displaying validation errors

I am using the last solution described in this problem  to read in a product price.
and my code is

C#
[VisibleInDetailView(true)]  
        [VisibleInListView(true)]  
        [ImmediatePostData]  
        [NotMapped]  
        [DisplayName("Product")]  
        public virtual Product DisplayProduct  
        {  
            get  
            {  
                return Product;  
            }  
            set  
            {  
                Product = value;  

                if (Product != null)  
                {  
                    var os = ObjectSpace;  
                    if (SalesHeader.PriceList == null)  
                    {  
                        throw new ExceptionDataValidation("uh uh price list is not entered");  
                    }  
                    var priceList = os.FindObject<PriceList>(CriteriaOperator.Parse("[Id]=? ", SalesHeader.PriceList.Id));  
                    if (priceList == null)  
                    {  
                        throw new ExceptionDataValidation("uh uh price list is not found");  
                    }  

                    Description = Product.Description;  
                    TaxRate = os.FindObject<TaxRate>(CriteriaOperator.Parse("[Id]=? ", HandyDefaults.DefaultTaxRate(os).Id));  

                    if (SalesHeader.PriceList.PriceListType == PriceListTypeEnum.DiscountFromRetailPrice)  
                    {  
                        Price = Product.RetailPrice * (100 - priceList.Percentage) / 100;  
                    }  
                    else  
                    {  
                        Price = Product.StandardCost * (100 + priceList.Percentage) / 100;  
                    }  

                    os.SetModified(this);  
                }  
            }  
        }  

My problem is that the error displays as " An exception has been thrown by the target of an invocation"

How do I get the desired exception message to be displayed?

I don't think validation rules will work because they only get checked on Saving.