1.cs

static string TryGetErrorMessage(XElement html)
{
    var errMessage = html.GetElementsByClassName("ErrorMessage").FirstOrDefault();
    var errText = html.GetElementsByClassName("ErrorText").FirstOrDefault();
    var errorMessage = String.Empty;
    if (errMessage != null && (!errMessage.Attributes("style").Any() || (errMessage.Attributes("style").Any() && errMessage.Attributes("style").First().Value != "visibility:hidden;")))
        errorMessage = errMessage.Value.Trim();
    if (errText != null)
        errorMessage = errText.Value.Trim();
    return errorMessage;
}

2.cs

static Maybe<string> TryGetErrorMessage(XElement html)
{
    return html.GetElementsByClassName("ErrorMessage").MaybeFirst().
        Where(x => !x.Attributes("style").Any() 
                || !x.Attribute("style").Value.Contains("visibility:hidden")).
        Or(html.GetElementsByClassName("ErrorText").MaybeFirst()).
        Select(x => x.Value.Trim());
}