curl_fetch_result: throw the right exception
curl_exception: record remote IP address and port
curl_fetch_result: include remote address in exceptions
symboldb_download_repo: report remote IP address in errors
symboldb: report remote IP address in errors
symboldb_download_repo: filter downloads against database even if not loading

This results in a reduction in network traffic if the RPM is in the
database, but not in the RPM cache.
symboldb_download_repo: pass functor state by reference

The counter increments in download_filter were lost because they
happened on a copy.