Added build error message for modules missing parent_select.module_id

Addresses: http://manage.dimagi.com/default.asp?133172#752523

A user managed to get a module into a state where `module.parent_select.active` was `True` but `module.parent_select.module_id` was `None`. This adds build validation that checks for that condition.
Since `parent_select` is not a property of `ModuleBase` this code should either check for its existence using `getattr` or else be moved to the subclass that defined the `parent_select` property.

Personally I think moving it to the `Module.validate_for_build` makes most sense.