on download failure, return cached version (if exists)

on download failure, return cached version (if exists)

Previously if a download failed, an error would be logged and
the rules that were downloaded were processed. This could lead
to an output that was no expected (missing rules).

Now if the download fails, check if we have the previous download.
If we do, log a warning and return the cached files.

If we don't have the previous download, log an error and process
what rules we do have.