[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.

mcs/class/System.XML/System.Xml.Schema/XmlSchemaException.cs

@@ -157,7 +157,7 @@ namespace System.Xml.Schema
 
 		private static string GetMessage (string message, string sourceUri, int lineNumber, int linePosition, XmlSchemaObject sourceObj)
 		{
-			string msg = "XmlSchema error: " + message;
+			string msg = message;
 			if (lineNumber > 0)
 				msg += String.Format (CultureInfo.InvariantCulture, " XML {0} Line {1}, Position {2}.",
 					(sourceUri != null && sourceUri != "") ? "URI: " + sourceUri + " ." : "",