pgsql: 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: /messages/by-id/20200925005531.GJ23631@telsasoft.com

Backpatch-through: 9.5

Branch
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REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/39c23c199d0aec58d0018a2ca52cbccbf8ea18e4

Modified Files
--------------
src/bin/pg_upgrade/check.c | 40 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 40 insertions(+)