One incompatible add-on no longer stops the portal starting

An add-on that runs a background task could stop the entire portal from starting if it had been built against a different version of the platform. Every new instance of the portal would fail at launch, so an update could not roll out at all until someone intervened by hand.

Now only the affected add-on is skipped, and the reason is reported. The portal starts and serves without that one feature instead of not starting at all.