FirstValidationErrorConverter.cs

public class FirstValidationErrorConverter : IValueConverter
	{
		public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
		{
			ICollection<string> errors = value as ICollection<string>;
			return errors != null && errors.Count > 0 ? errors.ElementAt(0) : null;
		}

		public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
		{
			return null;
		}
	}