Report roles in pg_upgrade pg_ prefix check

Looking at a recent pg_upgrade thread I happened to notice that the check for
roles with a pg_ prefix only reports the error, not the roles it found. Other
similar checks where the user is expected to alter the old cluster typically
reports the found objects in a textfile. The attached adds reporting to make
that class of checks consistent (the check for prepared transactions which also
isn't reporting is different IMO as it doesn't expect ALTER commands).

As this check is only executed against the old cluster the patch removes the
check when printing the error.

--
Daniel Gustafsson https://vmware.com/