MVC ModelError how to get value
ModelState.AddModelError("duplicateInvoiceNumberOrganisation", "The combination of organisation and invoice number must be unique");

controller.ModelState.Values

Count = 1
    [0]: {System.Web.Mvc.ModelState}

string error = ModelState["duplicateInvoiceNumberOrganisation"]
    .Errors[0]
    .ErrorMessage;

string error = ModelState
    .Where(x => x.Value.Errors.Count > 0)
    .First()
    .Value
    .Errors[0]
    .ErrorMessage;