private void onDocumentSelectionException(Exception e) {
        System.err.println("Illegal document selection string '" +
                params.getVisitorParameters().getDocumentSelection() + "'.\n");
        System.exit(1);
    }
If not specified in options, will get form cluster list
vespaclient-java/src/main/java/com/yahoo/vespavisit/VdsVisit.java