download: Handle temporary server errors gracefully (retry)

Modify our on-failure callback (which is called *every* time a package
fails, not after N retries) to permit retrying. URLGrabber will
blacklist the server after some number of failures, and Fedup will
finish downloading even in the face of some bad servers.

Set 'retries' on RepoConfig (grandparent of FedupDownloader) to five.