使用CQRS处理中介管道中的错误/异常

我正试图遵循Jimmy Bogard的这篇文章来实现一个中介管道，这样我就可以使用前/后请求处理程序来完成一些工作。从对那篇文章的评论中，我得出了github的要点。我还不太明白如何把这一切联系起来，所以这是我的第一次尝试。仅供参考-我正在为DI和Web Api 2使用Autofac。在CQRS之后，这里有一个查询。

public class GetAccountRequest : IAsyncRequest<GetAccountResponse>
{
    public int Id { get; set; }
}
//try using fluent validation
public class GetAccountRequestValidationHandler 
    : AbstractValidator<GetAccountRequest>, IAsyncPreRequestHandler<GetAccountRequest>
{
    public GetAccountRequestValidationHandler() {
        RuleFor(m => m.Id).GreaterThan(0).WithMessage("Please specify an id.");
    }
    public Task Handle(GetAccountRequest request) {
        Debug.WriteLine("GetAccountPreProcessor Handler");   
        return Task.FromResult(true);
    }
}
public class GetAccountResponse
{
    public int AccountId { get; set; }
    public string Name { get; set; }
    public string AccountNumber { get; set; }
    public string Nickname { get; set; }
    public string PhoneNumber { get; set; }
    public List<OrderAckNotification> OrderAckNotifications { get; set; }
    public class OrderAckNotification {
        public int Id { get; set; }
        public bool IsDefault { get; set; }
        public string Description { get; set; }
        public string Type { get; set; }
    }
}

GetAccountRequestHandler:

public class GetAccountRequestHandler 
    : IAsyncRequestHandler<GetAccountRequest, GetAccountResponse>
{
    private readonly IRedStripeDbContext _dbContext;
    public GetAccountRequestHandler(IRedStripeDbContext redStripeDbContext)
    {
        _dbContext = redStripeDbContext;
    }
    public async Task<GetAccountResponse> Handle(GetAccountRequest message)
    {
        //some mapping code here.. omitted for brevity
        Mapper.AssertConfigurationIsValid();
        return await _dbContext.Accounts.Where(a => a.AccountId == message.Id)
            .ProjectToSingleOrDefaultAsync<GetAccountResponse>();
    }

以下是当前显示HttpGet的webapi2控制器。

[RoutePrefix("api/Accounts")]
public class AccountsController : ApiController
{
    private readonly IMediator _mediator;
    public AccountsController(IMediator mediator)
    {
        _mediator = mediator;
    }
    // GET: api/Accounts/2
    [Route("{id:int}")]
    [HttpGet]
    public async Task<IHttpActionResult> GetById([FromUri] GetAccountRequest request)
    {
        var model = await _mediator.SendAsync<GetAccountResponse>(request);
        return Ok(model);
    }
}

最后是依赖解析代码：

public void Configuration(IAppBuilder app)
{
    var config = new HttpConfiguration();
    ConfigureDependencyInjection(app, config);
    WebApiConfig.Register(config);
    app.UseWebApi(config);
}
private static void ConfigureDependencyInjection(IAppBuilder app, 
    HttpConfiguration config)
{
    var builder = new ContainerBuilder();
    builder.RegisterSource(new ContravariantRegistrationSource());
    builder.RegisterAssemblyTypes(typeof(IMediator).Assembly).AsImplementedInterfaces();
    builder.Register<SingleInstanceFactory>(ctx =>
    {
        var c = ctx.Resolve<IComponentContext>();
        return t => c.Resolve(t);
    });
    builder.Register<MultiInstanceFactory>(ctx =>
    {
        var c = ctx.Resolve<IComponentContext>();
        return t => (IEnumerable<object>)c.Resolve(
            typeof(IEnumerable<>).MakeGenericType(t));
    });
    //register all pre handlers
    builder.RegisterAssemblyTypes(Assembly.GetExecutingAssembly())
        .As(type => type.GetInterfaces()
            .Where(t => t.IsClosedTypeOf(typeof(IAsyncPreRequestHandler<>))));
    //register all post handlers
    builder.RegisterAssemblyTypes(Assembly.GetExecutingAssembly())
        .As(type => type.GetInterfaces()
            .Where(t => t.IsClosedTypeOf(typeof(IAsyncPostRequestHandler<,>))));

    //register all handlers
    builder.RegisterAssemblyTypes(Assembly.GetExecutingAssembly())
        .As(type => type.GetInterfaces()
            .Where(t => t.IsClosedTypeOf(typeof(IAsyncRequestHandler<,>)))
            .Select(t => new KeyedService("asyncRequestHandler", t)));
    //register pipeline decorator
    builder.RegisterGenericDecorator(typeof(AsyncMediatorPipeline<,>), 
        typeof(IAsyncRequestHandler<,>), "asyncRequestHandler");
    // Register Web API controller in executing assembly.
    builder.RegisterApiControllers(Assembly.GetExecutingAssembly()).InstancePerRequest();
    //register RedStripeDbContext
    builder.RegisterType<RedStripeDbContext>().As<IRedStripeDbContext>()
        .InstancePerRequest();
    builder.RegisterType<AutofacServiceLocator>().AsImplementedInterfaces();
    var container = builder.Build();
    config.DependencyResolver = new AutofacWebApiDependencyResolver(container);
    // This should be the first middleware added to the IAppBuilder.
    app.UseAutofacMiddleware(container);
    // Make sure the Autofac lifetime scope is passed to Web API.
    app.UseAutofacWebApi(config);
}

我正在进入GetAccountRequestValidationHandler。但是，当验证失败(传递了0的id(时，我如何抛出异常或停止管道的执行？如何返回.WithMessage？