/// <summary>
        /// Register external error not possible to check with attributes
        /// </summary>
        /// <param name="instance">The object instance.</param>
        /// <param name="validatorProperty">The property to register.</param>
        /// <param name="errorMessage">The error message.</param>
        /// <returns></returns>
        protected void RegisterValidationError(object instance, string validatorProperty, string errorMessage)
        {
            ErrorSummary summary = GetErrorSummary(instance);

            summary.RegisterErrorMessage(validatorProperty, errorMessage);

            validationSummaryPerInstance[instance] = summary;
        }