This is an issues one of my friends noticed that dundjini was down for one reason or another.
It seems like a terribly simple problem to have external sources not be filtered out if they go down for some reason. Maybe the client could test if a given site can be reached error free before giving resources from it, and if a site is down then that site would be filtered out of the searches for a few minutes for that GM, so he doesn't see a bunch of unusable assets nor would he ping them like crazy.