Propagate exception object when error occurs

[zzattack]

Users (like me) may be interested to know the underlying exception when some error occurs. This patch makes ErrorEventArgs expose a field with this exception, if it's available.

[sta]

It looks nice.

And please:

• Add a constructor.

    internal ErrorEventArgs (string message)
      : this (message, null)
    {
    }

    internal ErrorEventArgs (string message, Exception exception)
    {
      _message = message;
      _exception = exception;
    }

It's because optional parameters are available in C# 4.0 and later.

• Add more xml doc comments to the Exception property, like the following.

    /// <summary>
    /// Gets the exception that caused the error.
    /// </summary>
    /// <value>
    /// A <see cref="Exception"/> instance that represents the cause of the error,
    /// or <see langword="null"/> if the error isn't due to an exception.
    /// </value>
    public Exception Exception {
    ...

[zzattack]

As requested!

[sta]

I will merge this PR, and then add the following comment to those .cs files.

/*
 * Contributors:
 * - Frank Razenberg <frank@zzattack.org>
 */

Is it okay?

[zzattack]

Absolutely, thanks for picking this up. No worries about the comment.