How to return error from ASP.NET MVC action

I am using ASP.NET MVC for developing a web site. I am using jquery for AJAX functionality. In the action methods, I want to return some error to signal that the input is not correct or that the action could not be performed. In such error cases, I expect the jquery ajax error handler to be called and I can take appropriate action in there. I have not found a way how to do this. Following is my action method.

In error cases, what should I be sending from an Action in order to get the jquery error handler triggered?

public ActionResult AddToFavourites(int entityId, string entityType)
    {

        if (!Request.IsAjaxRequest())
            throw new InvalidOperationException("This action can be called only in async style.");

        try
        {
            RBParams.EntityType typeOfFavourite = (RBParams.EntityType)Enum.Parse(typeof(RBParams.EntityType), entityType);
            string status = "";

            if (typeOfFavourite == RBParams.EntityType.BusinessEntity)
            {
                status = MarkFavouriteEntity(entityId);
            }
            else if (typeOfFavourite == RBParams.EntityType.Review)
            {
                status = MarkFavouriteReview(entityId);
            }
            else
            {
                throw new InvalidOperationException("The type of the entity is not proper");
            }

            return Content(status);

        }
        catch (Exception ex)
        {

            return Content("Error");
        }
    }

Your ajax error handler will be called when the action doesn't return a expected status code. It will, for example, fire if the action wasn't found or if you throw a exception that you don't handle. In your case it will be called if you don't catch the error in your action (as the action will return a 500 status code).

I would, however, not do it in this way as this is probably a expected error. I would rather return json both when you succeed and when you have a error. Then you can indicate if it is a successful call or not. Something like this:

public ActionResult AddToFavourites(int entityId, string entityType)
{

    if (!Request.IsAjaxRequest())
        throw new InvalidOperationException("This action can be called only in async style.");

    try
    {
        RBParams.EntityType typeOfFavourite = (RBParams.EntityType)Enum.Parse(typeof(RBParams.EntityType), entityType);
        string status = "";

        if (typeOfFavourite == RBParams.EntityType.BusinessEntity)
        {
            status = MarkFavouriteEntity(entityId);
        }
        else if (typeOfFavourite == RBParams.EntityType.Review)
        {
            status = MarkFavouriteReview(entityId);
        }
        else
        {
            throw new InvalidOperationException("The type of the entity is not proper");
        }

        return Json(new { Success = true, Status = status });

    }
    catch (Exception ex)
    {

        return Json(new { Success = false, Message = ex.Message });
    }
}

Then you handle it in the same way as a successful call. You just check the Success property of your json response. Then you handle unexpected errors in the error callback.

Mattias Jakobsson's answer is right. But I think the best way to return the error to the jQuery is creating a JSON and sending it with status 500. However, when I did it and tried to deploy my MVC website using IIS 7 I figured out that it was returning me the custom page instead of the message.

The code was...

catch (Exception ex)
{
    Response.StatusCode = 500;
    return Json(new { error = ex.Message });
}

But then I saw this thread that led me to this web site (from Rick Strahl).

Overall, I understood that you need to say to IIS to not inject the custom error page, so you need this flag (in the global.asax or into the catch):

Response.TrySkipIisCustomErrors = true;

So, in jQuery the code keeps the same:

$.ajax({...})
.done(function (result) {...})
.fail(function (e) { 
   console.log(e.responseJSON.error);
});

you shoud config jquery to handle error:

$.ajaxSetup({
    error: function(xhr) {
        alert(xhr.statusText);
    }
})