title: Add error message to @Html.ValidationSummary
body: <p>I'm using standard MVC3 Razor views with unobtrusive Javascript validation, using <code>@Html.ValidationSummary</code> to show them at the top of the form. If the standard validations (things like <code>[Required]</code>) pass, I then run some very custom client-side validation that fires when the user hits the Submit button. (The validation looks across a number of form elements to make sure that the proper set of them have been checked, etc., so it's not as simple as just creating a new custom validator for a single field).</p>
  
  <p>I'd like the possible error(s) I construct there to be shown in the <code>ValidationSummary</code> list, but I can't figure out how to get the error message to appear there. </p>