title: Validation errors not disappearing after correction
body: <p>My MVC Model is like this 2 required fields and 2 optional required fields [ If HasOptional property is true , then only last 2 fields are mandatory]</p>
  
  <pre><code>[Required(ErrorMessage="Required...")]
  public string Title { get; set; }
  [Required(ErrorMessage="Required...")]
  public string Publisher { get; set; }
  
  public bool HasOptional{get;set;}
  
  public string OptTitle { get; set; }
  public string OptPublisher { get; set; }
  </code></pre>
  
  <p>So in Controller POST action i did was</p>
  
  <pre><code>if(model.HasOptional)
  {
      if (string.IsNullOrEmpty(Convert.ToString(model.OptTitle)))
      {
        ModelState.AddModelError.Add("OptTitle","required....");
      }
      if (string.IsNullOrEmpty(Convert.ToString(model.OptPublisher)))
      {
        ModelState.AddModelError.Add("OptPublisher","required....");
      }
  }
  </code></pre>
  
  <p>And if the property is null i gets the Validation messages in view back without any issues.But my question is 
  If i am filling OptTitle / OptPublisher in View and change the focus of textbox , still i can see the validation messages on screen . <strong>Is  there any way to disappear the Validation after filling the info [ like the default behaviour for first 2 properties  Title and Publisher]</strong></p>