Forward KNS installer error (so user is informed of their options)
As discovered by Piotr, attempting to update some KNewStuff originated packages in Discover results in the update not in fact being updated, without anything happening in Discover apart from the update just staying around in the UI. This patch forwards an error from KNSCore::Engine which gives some further information as to what the user can do to perform this update.
This is a half-solution to the problem, as the TODO added in here suggests, but it seems to me better to be explicit about the problem to our users, rather than just not doing anything, and informing them how they can fix it themselves. Until we have a full query system for Discover's transactions, this seems likely to be the best we can do (and actually implementing that system will take some time to get right, and we don't want to just ignore the problem until them).