[ServiceContract(Namespace = "")]
    [AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
    // в web.confi-e задано : <serviceDebug includeExceptionDetailInFaults="true" />
    // [ServiceBehavior(IncludeExceptionDetailInFaults=true)]
    public class MySrv
    {
        [OperationContract]
        [FaultContract(typeof(ErrInfo))]
        public string ErrTest(int id)
        {
            string rv = string.Empty;

            if (id == 1)
            {
                ErrInfo ei = new ErrInfo() { Message = "ErrInfo message", Detail = "ErrInfo Detail" };
                throw new FaultException<ErrInfo>(ei, "Some reason");
            }

            rv = "success " + id.ToString();

            return rv;
        }
    }

    [DataContract]
    public class ErrInfo
    {
        [DataMember]
        public string Message { get; set; }
        [DataMember]
        public string Detail { get; set; }
    }

        void cl_ErrTestCompleted(object sender, srv.ErrTestCompletedEventArgs e)
        {
            string rv = string.Empty;

            if (e.Error != null)
            {
                // почему тип возвращаемой ошибки не FaultException<srv.ErrInfo>?
                if (e.Error is FaultException<srv.ErrInfo>)
                {
                    FaultException<srv.ErrInfo> ei = e.Error as FaultException<srv.ErrInfo>;
                    rv = string.Format("{0}\r\n{1}\r\n{2}", ei.Reason, ei.Detail.Message, ei.Detail.Detail);
                }
                else
                    rv = e.Error.Message;
            }
            else
                rv = e.Result;

            this.textBox2.Text = rv;
        }

 <serviceDebug includeExceptionDetailInFaults="false/true" />

bool registerResult = WebRequest.RegisterPrefix("http://", WebRequestCreator.ClientHttp);