fix for #707

using the blog entry from Josh Smith
Binding to (Validation.Errors)[0] without Creating Debug Spew

http://joshsmithonwpf.wordpress.com/2008/10/08/binding-to-validationerrors0-without-creating-debug-spew/
Merge pull request #719 from MahApps/707-punker76-validation-binding-errors

fix for #707 errors in ValidationToolTipTemplate