[Sys.Xml] Add unit test for Novell bug #599689

The unit test demonstrates a difference between Mono and .NET
(https://bugzilla.novell.com/show_bug.cgi?id=599689).
[Sys.Xml] Fix Novell bug #599689

This change fixes #599689
(https://bugzilla.novell.com/show_bug.cgi?id=599689) by making the
output of XmlSchemaException.ToString() match the output under .NET.
Merge pull request #922 from ermshiperete/novell-bug-599689

Fix Novell bug 599689