Improve error handling during replication download

[lonvia]

The replication handler currently catches all errors that happen during download of diff files and ends the download process as if it was finished. The idea behind this is that a continuous script will simply want to retry on transient errors. However there are some permanent errors that really should be reported back as such. A 404 on a state or diff file would be such a condition.

See also osm2pgsql-dev/osm2pgsql#2206.