title: Accessing ErrorMessage from within custom validation attribute
body: <p>I want to be able to read the <code>ErrorMessage</code> string passed into a custom validation attribute so I can do some <code>string.Format()</code>ing on it. But I can't seem to find it?</p>
  
  <p>So for example I have a custom attribute:</p>
  
  <pre><code>[IsDateBeforeFixedDate(4, 0, 0, ErrorMessage = "*The departure date should be between 4 days and 11 months")]
  </code></pre>
  
  <p>decalred as:</p>
  
  <pre><code>  public sealed class IsDateBeforeFixedDateAttribute : ValidationAttribute, IClientValidatable
    {
  
      public IsDateBeforeFixedDateAttribute(int days, int months, int years) : base(days, months, years)
      {
        //I want to read the ErrorMessage string here! (i.e. "*The departure date should be between 4 days and 11 months")
      }
    }
  </code></pre>
  
  <p><code>ErrorMessageString</code> doesn't contain it or <code>ErrorMessage</code></p>