How can I change OAuthAuthorizationServerProvider HttpResponse status code from 400 to my own?

Question

My app is default web-api on C#, that uses Microsoft.Owin.Security.OAuth for user authentication. I'm building my own OAuthAuthorizationServerProvider implementation. My GrantResourceOwnerCredentials method looks like:

    public override async Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
            {
                //this method searches for user in database
                var user = await accountService.LoginAsync(context.UserName, context.Password);

                if (user != null)
                {
                    if (user.Password == "-1")
                    {
                        //here i want to set different HTTPStatusCode for user that is not registered or has wrong password

                        //context.Response.StatusCode = (int)HttpStatusCode.Forbidden;
                        //context.Response.Context.Response.StatusCode = 403;
                        //context.SetError("403", "password not valid");
                        //context.Rejected();
                        return;
                    }
                }
                else if (user == null)
                {
                    //context.Response.Headers.Add(CoreConfiguration.OwinChallengeFlag, new[] { ((int)HttpStatusCode.NotFound).ToString() });
                    //context.Rejected();
                    return;
                }


//*here goes ClaimsIdentity and AuthenticationTicket creation*

So my question is: how can I customise my HttpStatusCode for response in different situations? I know that i can write my own middleware like this, but i don't believe that it is the only way.

The problem is that OAuthGrantResourceOwnerCredentialsContext have no any influence on actual HttpRespose, because, as I understand, this context can be only valid or not. Depending on this, OAuthAuthorizationServerProvider creates HttpResponse with 200 or 400 status code.

Also I know that for user validation exist ValidateClientAuthentication method, but i did not find any ways to customise StatusCode from it's OAuthValidateClientAuthenticationContext too.

Please explain me where am I wrong and whether it is possible to achieve my goal?