libtracker-miner: Proceed to next folder in root after errors

When getting error on a folder, we just want to grind the index root indexing to a halt when interrupted. Jumping directly to the next root is a weird middle ground in that it goes ahead and notifies as if finished, but then proceeds checking the missing folders.

Let it maybe continue to the next folder in the root (unless interrupted) so the whole index root is indexed (minus the failing folder).