model - bug fix for no callback exists
model - bug fix for no callback exists

If a callback and an extension are specified and the extension doesn't
exist, a JS error would occur.  Error no longer occurs and now a warning
is thrown to the console.

campuscolors/campuscolors,speedaddictcycles/speedaddictcycles,campuscolors/campuscolors,speedaddictcycles/speedaddictcycles,campuscolors/campuscolors,CommerceRack/anycommerce,CommerceRack/anycommerce,speedaddictcycles/speedaddictcycles,sportsworldchicago/app.sportsworldchicago.com---AnyCommerce-Development,sportsworldchicago/app.sportsworldchicago.com---AnyCommerce-Development,CommerceRack/anycommerce