Encapsulate the location of a ValidationEvent.
The ValidationEventLocator indicates where the ValidationEvent
occurred. Different fields will be set depending on the type of
validation that was being performed when the error or warning was detected.
For example, on-demand validation would produce locators that contained
references to objects in the Java content tree while unmarshal-time
validation would produce locators containing information appropriate to the
source of the XML data (file, url, Node, etc).