MODx.combo:   Notify on error while loading store

Currently,  if the store loading fails nothing happens.    The user has no way to know why (*unless they open dev-tools and try to find the response...*).    This patch will pop-up a notification.

In our case,  we have a combo which is populated via third-party API endpoint.   with  apikey, authentication token and so on.. many things can "normally" go wrong.