I can give you a real world example. The Html Agility Pack library is a parsing library. It simply defines a list of parsing errors on the the reader class. Extending your example, it would be something like:

    public abstract class ModelReader
    {
        private List<ParseError> _errors = new List<ParseError>();
        private bool _throwOnError;

        public ModelReader()
           :this(true)
        {
        }

        public ModelReader(bool throwOnError)
        {
           _throwOnError = throwOnError;
        }

        // use AddError in implementation when an error is detected
        public abstract Model Read();

        public virtual IEnumerable<ParseError> Errors
        {
           get {return _errors;}
        }

        protected virtual void AddError(ParseError error)
        {
           if (_throwOnError) // fail fast?
              throw new ParseException(error);

           _errors.Add(error);
        }
    }

    public class ParseError
    {
        public ParseError(...)
        {
        }

        public ParseErrorCode Code { get; private set; }
        public int Line { get; private set; }
        public int LinePosition { get; private set; }
        public string Reason { get; private set; }
        public string SourceText { get; private set; }
        public int StreamPosition { get; private set; }
    }

    public enum ParseErrorCode
    {
       InvalidSyntax,
       ClosingQuoteNotFound,
       ... whatever...
    }

    public class ParseException: Exception
    {
        ...
    }

And you still can add an event if the library caller wants on-the-fly events.