Avoid bombing out of loaddata if all the pages aren't there yet

When importing a dump with loaddata, it may fail due to calling generate_menu before all the pages related to a menu are loaded. This makes that failure non-fatal during loading, on the assumption that the final call to refresh_menu_cache will sort everything out correctly.
Maybe this should be ``kwargs.get('raw')``? Or we should just explicitly list raw in the parameter list?