Promote method with a possibility to fail while saving data to the DB

Promote.cs

[HttpPost]
[Route("customers/{id}/promotion")]
public HttpResponseMessage Promote(long id)
{
    return _customerRepository.GetById(id)
        .ToResult("Customer with such Id is not found: " + id)
        .Ensure(customer => customer.CanBePromoted(), "The customer has the highest status possible")
        .OnSuccess(customer => customer.Promote())
        .OnSuccess(customer => _unitOfWork.Flush().Map(() => customer))
        .OnSuccess(customer => _emailGateway.SendPromotionNotification(customer.PrimaryEmail, customer.Status)
            .OnFailure(() => _unitOfWork.RollBack()))
        .OnBoth(result => result.IsSuccess ? Ok() : Error(result.Error));
}