Throw error for mismatched pg_upgrade clusters

If someone removes the 'postgres' database from the old cluster and the
new cluster has a 'postgres' database, the number of databases will not
match.  We actually could upgrade such a setup, but it would violate the
1-to-1 mapping of database counts, so we throw an error instead.

Previously they got an error during the upgrade, and not at the check
stage; PG 9.0.4 does the same.