pg_upgrade: generate check error for left-over new tablespace

pg_upgrade:  generate check error for left-over new tablespace

Previously, if pg_upgrade failed, and the user recreated the cluster but
did not remove the new cluster tablespace directory, a later pg_upgrade
would fail since the new tablespace directory would already exists.
This adds error reporting for this during check.

Reported-by: Justin Pryzby

Discussion: https://postgr.es/m/20200925005531.GJ23631@telsasoft.com

Backpatch-through: 9.5