bad date literals break JSON rendering

[epimorphics]

bad date literals break JSON rendering

[ehedgehog]

I believe it's the econde line below (from RDFUtil):
public static String formatDateTime(Literal l) {
        Object val = l.getValue();
        if (val instanceof XSDDateTime) {
            boolean isDate = l.getDatatype().equals(XSDDatatype.XSDdate);
            Date date = ((XSDDateTime)val).asCalendar().getTime();
            return dateFormat(hasTimeZone(l.getLexicalForm()), isDate).format(date);
        } else {
            return null;
        }
     }
Tricky because we'd prefer a proper JS date to come out. But instead I expect
we'll generate an error value of some kind.