title: Fluent validation: set custom message on custom validation
body: <p><strong>Scenario</strong></p>
  
  <p>I have a custom rule to validate the shipping cost of an order:</p>
  
  <pre><code>public class OrderValidator : BaseValidator&lt;Order&gt;
  {
  
      private string CustomInfo { get; set; }
  
      public OrderValidator()
      {
          //here I call the custom validation method and I try to add the CustomInfo string in the message
          RuleFor(order =&gt; order.ShippingCost).Cascade(CascadeMode.StopOnFirstFailure).NotNull().Must(
              (order, shippingCost) =&gt; CheckOrderShippingCost(order, shippingCost)
          ).WithMessage("{PropertyName} not set or not correct: {PropertyValue}." + (String.IsNullOrEmpty(CustomInfo) ? "" : " " + CustomInfo));
      }
  
      //this is the custom validation method
      private bool CheckOrderShippingCost(Order o, decimal shippingCost)
      {
          bool res = false;
  
          try
          {
              /*
               * check the actual shippingCost and set the res value
               */
          }
          catch (Exception ex)
          {
              CustomInfo = ex.ToString();
              res = false;
          }
  
          return res;
      }
  }
  </code></pre>
  
  <p>In case of exception, I store the exception info into the CustomInfo private member and I add it to the validation message.</p>
  
  <p>Then I run the validator:</p>
  
  <pre><code>OrderValidator oVal = new OrderValidator();
  oVal.Results = oVal.Validate(order);
  if (!oVal.Results.IsValid)
      oVal.Results.Errors.ForEach(delegate(ValidationFailure error) {
          Console.WriteLine(error.ErrorMessage);
      });
  </code></pre>
  
  <p><strong>Issue</strong></p>
  
  <p>Everything works right, in case of exception the CustomInfo is properly set to the ex.ToString() value. But eventually the error message displayed in the console does NOT show the CustomInfo, but only the first part of the message:</p>
  
  <pre><code>      "Shipping Cost not set or not correct: 5.9"
  </code></pre>
  
  <p><strong>Question</strong></p>
  
  <p>Why the custom message does not contains the CustomInfo string?
  Is it possible to add the exception info the the custom message in another way?</p>