title: Web Api - how to stop the web pipeline directly from an OnActionExecuting Filter
body: <p>I have a pre-action web api hook that will check ModelState.IsValid. If the ModelState is not valid I do not want to execute the action and just return my message immediately. How exactly do I do this?</p>
  
  <pre><code>public class ValidateModelStateAttribute : ActionFilterAttribute
  {
      public override void OnActionExecuting(System.Web.Http.Controllers.HttpActionContext actionContext) {
          if (!actionContext.ModelState.IsValid)
          {
              var msg = actionContext.Request.CreateErrorResponse(HttpStatusCode.BadRequest, actionContext.ModelState);
              // Now What?
          }
          base.OnActionExecuting(actionContext);
      }
  }
  </code></pre>