Nectar

The lazy download tasks check to see if files are present on disk when the `download_started` event is called. I'd like to be able halt a download in the `download_started` event if, say, the file exists and its checksum is what I expect. I would like to do this by raising some exception (SkipLocation looks like a likely candidate, though it appears to be used when the location isn't reachable, so maybe it's not appropriate.

The reason this is currently impossible is that the `download_started` call happens outside of the big try block: https://github.com/pulp/nectar/tree/5559de52c23e89a35d80febe38981c352a9cf5b0/nectar/downloaders/threaded.py#L205