pgsql: Fail REPACK in presence of invalid indexes, take 2

Fail REPACK in presence of invalid indexes, take 2

Commit 0c5d6269614e was confused about which indexes are potentially
problematic to rebuild: we thought only !indisready indexes could fail
to rebuild, and thus would attempt to rebuild !indisvalid ones that are
"ready" (that is, being maintained by DML but not yet completed).
However, that's wrong, because it could be that the validation phase of
such an index failed or was interrupted partway through. So, have
REPACK throw an error with all !indisvalid indexes, not just those
!indisready.

(!indisready implies !indisvalid, so this is an expansion of what
happened with the previous commit.)

Reported-by: shihao zhong <zhong950419(at)gmail(dot)com>
Backpatch-through: 19
Discussion: https://postgr.es/m/CAGRkXqRA4YetT8Nu3+1-45+ffTwi2bd1d1eDFME39Y9OPRMytQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/78142821dfd42a7ca1010811ce77ff7b96218741

Modified Files
--------------
doc/src/sgml/ref/repack.sgml | 6 ++++++
src/backend/commands/repack.c | 15 ++++++++-------
2 files changed, 14 insertions(+), 7 deletions(-)