SimplerActionFilter.cs

public class ModelBindingFailureFilterSimple : IActionFilter
{
    public void OnActionExecuted(ActionExecutedContext context)
    {
    }

    public void OnActionExecuting(ActionExecutingContext context)
    {
        if (!context.ModelState.IsValid)
        {
            context.Result = new BadRequestObjectResult(
                context
                .ModelState
                .ToDictionary(
                    x=>x.Key, 
                    y=>y.Value.Errors.Select(
                        z=>z.Exception.Message).ToList()));
        }
    }
}