title: Stop Fluent Validation on first failure
body: <p>i'm defining a validation for my Request objects.
  I would like the validator to stop on the very first failure, not only the one on the same chain.
  In the example below, if my <code>TechnicalHeader</code> object is null, i get a NullReference exception when the validation reaches the rule for <code>TechnicalHeader.MCUserid</code>.</p>
  
  <p>In poor words, i would like to do a conditional validation over the last three rules in the code below, according to the result of the first rule</p>
  
  <pre><code>using System;
  using ServiceStack.FluentValidation;
  using MyProj.Services.Models;
  
  namespace MyProj.Services.BaseService.Validators
  {
      public class BaseValidator&lt;T&gt; : AbstractValidator&lt;T&gt;
          where T : RequestBase
      {
          public BaseValidator()
          {
              RuleSet(ServiceStack.ApplyTo.Put | ServiceStack.ApplyTo.Post, 
                  () =&gt;
                  {
                      this.CascadeMode = CascadeMode.StopOnFirstFailure;
                      RuleFor(x =&gt; x.TechnicalHeader).Cascade(CascadeMode.StopOnFirstFailure).NotNull().WithMessage("Header cannot be null");
                      RuleFor(x =&gt; x.TechnicalHeader).NotEmpty().WithMessage("Header cannot be null");
                      RuleFor(x =&gt; x.TechnicalHeader.Userid).NotEmpty().WithMessage("Userid cannot be null or an empty string");
                      RuleFor(x =&gt; x.TechnicalHeader.CabCode).GreaterThan(0).WithMessage("CabCode cannot be or less than 0");
                      RuleFor(x =&gt; x.TechnicalHeader.Ndg).NotEmpty().WithMessage("Ndg cannot be null or an empty string");
                  }
              );
          }
      }
  }
  </code></pre>