Kendo: Handling Errors in Ajax Data Requests

If you need to display an error message from the server then you can do it by returning a DataSourceResult object with only its Errors property set:

return this.Json(new DataSourceResult
            {
                Errors = "my custom error"
            });

And pick it up on the client by using this (referenced by the .Events(events => events.Error("onError")) line):

function onError(e, status) {
    if (e.status == "customerror") {
        alert(e.errors);
    }
    else {
        alert("Generic server error.");
    }
}