When installing an incompatible add-on, the add-ons manager checks first if a newer and compatible version of that add-on is available by sending a request either to the AUS or the provided update URL in the manifest.
If there's no update URL in the manifest and if the application does not provide an add-on update URL via preferences, the add-ons manager will error out and fail to notify that the said add-on is incompatible.
This commit addresses that by:
(a) preventing substitutions on the update manifest URL - this throws an error if it's empty; and
(b) failing early in the add-on update checker if the update manifest URL is empty and sends out an error notification
