The IIOInvalidTreeException class subclasses IIOException, which provides
a constructor of the form (String message, Throwable cause). In the case
where an exception occurs while attempting to parse a node, it would be
desirable to attach the underlyin exception to the IIOInvalidTreeException.
Although this can be accomplished by first constructing the exception and
then calling initCause, it would be cleaner to provide a constructor
that includes a Throwable argument, which would be passed up to the
superclass constructor.