postgresql dump and load -- need to dump "postgres" db also?
The postgres database is just the default database that is created during initdb. It has no special meaning. 

If you created a different one where all your data is kept and you never created tables in the postgres database, then there is no need to dump it.

I don't know what pg_upgradecluster is. The default tool to upgrade the data directory (apart from dump/restore) is pg_upgrade. 

If you want help with the upgrade, "got a lot of error" won't get you much help. You need to be a lot more detailed.