Fix defaultDate option of DatePicker when including TimePicker

Just including TimePicker breaks DatePickers defaultDate option, as it swallows all exceptions generated by $.datepicker.parseDate.

DatePicker relies on certain exceptions being thrown by that method, though. This fix double-checks, and only swallows exceptions which are actually handled by the additional code in TimePicker's defaultDate function.

This fixes #546 and probably #270, too