FdoDateTime: Avoid returning invalid DateTime values

[gregboone]

When using the FDO Expression Engine or when accessing FDO data stores using the FDO managed API, there exists a slight possibility that:

1) a corrupted date (year == 0) may be returned from the expression engine

2) an exception may be thrown when reading an unmanged FdoDateTime value and converting it to a System.DateTime value

For the Expression Engine, exceptions are thrown for invalid months/days/etc... so an invalid year should also be handled in teh same manner. It is a long term goal that the EE become more forgiving whebn handling DateTime values, but that llarger change is not currently in scope for FDO 3.8.

For the FDO Managed API, the date conversion typically happens when reading date values using the FdoIReader interface. Throwing exceptions when reading invalid datstore dates can be fatal to an application and in retropsect does not need to be treated as a hard failure.