如何获取DotNetOpenAuth返回的错误信息.客户端OAuth2

我使用ExchangeUserCredentialForToken函数从授权服务器获取令牌。当我的用户存在于我的数据库中时，它工作得很好，但是当凭据不正确时，我想向客户端发回一条消息。我使用以下两行代码来设置错误消息:

context.SetError("Autorization Error", "The username or password is incorrect!");
context.Rejected();

但是在客户端，我只得到协议错误(错误400)。你能帮我如何在授权服务器上获得服务器端设置的错误消息吗?

来自授权服务器的完整应用配置:

using Constants;
using Microsoft.Owin;
using Microsoft.Owin.Security;
using Microsoft.Owin.Security.Cookies;
using Microsoft.Owin.Security.Infrastructure;
using Microsoft.Owin.Security.OAuth;
using Owin;
using System;
using System.Collections.Concurrent;
using System.Linq;
using System.Security.Claims;
using System.Security.Principal;
using System.Threading.Tasks;
using AuthorizationServer.Entities;
using AuthorizationServer.Entities.Infrastructure.Abstract;
using AuthorizationServer.Entities.Infrastructure.Concrete;
namespace AuthorizationServer
{
    public partial class Startup
    {
        private IEmployeeRepository Repository;  
        public void ConfigureAuth(IAppBuilder app)
        {
            //instanciate the repository
            Repository = new EmployeeRepository();
            // Enable Application Sign In Cookie
            app.UseCookieAuthentication(new CookieAuthenticationOptions
            {
                AuthenticationType = "Application",
                AuthenticationMode = AuthenticationMode.Passive,
                LoginPath = new PathString(Paths.LoginPath),
                LogoutPath = new PathString(Paths.LogoutPath),
            });
            // Enable External Sign In Cookie
            app.SetDefaultSignInAsAuthenticationType("External");
            app.UseCookieAuthentication(new CookieAuthenticationOptions
            {
                AuthenticationType = "External",
                AuthenticationMode = AuthenticationMode.Passive,
                CookieName = CookieAuthenticationDefaults.CookiePrefix + "External",
                ExpireTimeSpan = TimeSpan.FromMinutes(5),
            });
            // Enable google authentication
            app.UseGoogleAuthentication();
            // Setup Authorization Server
            app.UseOAuthAuthorizationServer(new OAuthAuthorizationServerOptions
            {
                AuthorizeEndpointPath = new PathString(Paths.AuthorizePath),
                TokenEndpointPath = new PathString(Paths.TokenPath),
                ApplicationCanDisplayErrors = true,
#if DEBUG
                AllowInsecureHttp = true,
#endif
                // Authorization server provider which controls the lifecycle of Authorization Server
                Provider = new OAuthAuthorizationServerProvider
                {
                    OnValidateClientRedirectUri = ValidateClientRedirectUri,
                    OnValidateClientAuthentication = ValidateClientAuthentication,
                    OnGrantResourceOwnerCredentials = GrantResourceOwnerCredentials,
                    OnGrantClientCredentials = GrantClientCredetails
                },
                // Authorization code provider which creates and receives authorization code
                AuthorizationCodeProvider = new AuthenticationTokenProvider
                {
                    OnCreate = CreateAuthenticationCode,
                    OnReceive = ReceiveAuthenticationCode,
                },
                // Refresh token provider which creates and receives referesh token
                RefreshTokenProvider = new AuthenticationTokenProvider
                {
                    OnCreate = CreateRefreshToken,
                    OnReceive = ReceiveRefreshToken,
                }
            });
            // indicate our intent to use bearer authentication
            app.UseOAuthBearerAuthentication(new OAuthBearerAuthenticationOptions
            {
                AuthenticationType = "Bearer",
                AuthenticationMode = Microsoft.Owin.Security.AuthenticationMode.Active
            });
        }
        private Task ValidateClientRedirectUri(OAuthValidateClientRedirectUriContext context)
        {
            if (context.ClientId == Clients.Client1.Id)
            {
                context.Validated(Clients.Client1.RedirectUrl);
            }
            else if (context.ClientId == Clients.Client2.Id)
            {
                context.Validated(Clients.Client2.RedirectUrl);
            }
            return Task.FromResult(0);
        }
        private Task ValidateClientAuthentication(OAuthValidateClientAuthenticationContext context)
        {
            string clientname;
            string clientpassword;

            if (context.TryGetBasicCredentials(out clientname, out clientpassword) ||
                context.TryGetFormCredentials(out clientname, out clientpassword))
            {
                employee Employee = Repository.GetEmployee(clientname, clientpassword);
                if (Employee != null)
                {
                    context.Validated();
                }
                else
                {
                    context.SetError("Autorization Error", "The username or password is incorrect!");
                    context.Rejected();
                }
            }
            return Task.FromResult(0);
        }
        private Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
        {
            var identity = new ClaimsIdentity(new GenericIdentity(context.UserName, OAuthDefaults.AuthenticationType), context.Scope.Select(x => new Claim("urn:oauth:scope", x)));
            context.Validated(identity);
            return Task.FromResult(0);
        }
        private Task GrantClientCredetails(OAuthGrantClientCredentialsContext context)
        {
            var identity = new ClaimsIdentity(new GenericIdentity(context.ClientId, OAuthDefaults.AuthenticationType), context.Scope.Select(x => new Claim("urn:oauth:scope", x)));
            context.Validated(identity);
            return Task.FromResult(0);
        }

        private readonly ConcurrentDictionary<string, string> _authenticationCodes =
            new ConcurrentDictionary<string, string>(StringComparer.Ordinal);
        private void CreateAuthenticationCode(AuthenticationTokenCreateContext context)
        {
            context.SetToken(Guid.NewGuid().ToString("n") + Guid.NewGuid().ToString("n"));
            _authenticationCodes[context.Token] = context.SerializeTicket();
        }
        private void ReceiveAuthenticationCode(AuthenticationTokenReceiveContext context)
        {
            string value;
            if (_authenticationCodes.TryRemove(context.Token, out value))
            {
                context.DeserializeTicket(value);
            }
        }
        private void CreateRefreshToken(AuthenticationTokenCreateContext context)
        {
            context.SetToken(context.SerializeTicket());
        }
        private void ReceiveRefreshToken(AuthenticationTokenReceiveContext context)
        {
            context.DeserializeTicket(context.Token);
        }
    }
}