pgsql: Promote assertion about !ReindexIsProcessingIndex to runtime err

Promote assertion about !ReindexIsProcessingIndex to runtime error.

When this assertion was installed (in commit d2f60a3ab), I thought
it was only for catching server logic errors that caused accesses to
catalogs that were undergoing index rebuilds. However, it will also
fire in case of a user-defined index expression that attempts to
access its own table. We occasionally see reports of people trying
to do that, and typically getting unintelligible low-level errors
as a result. We can provide a more on-point message by making this
a regular runtime check.

While at it, adjust the similar error check in
systable_beginscan_ordered to use the same message text. That one
is (probably) not reachable without a coding bug, but we might as
well use a translatable message if we have one.

Per bug #18363 from Alexander Lakhin. Back-patch to all supported
branches.

Discussion: https://postgr.es/m/18363-e3598a5a572d0699@postgresql.org

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/940489b46769eb2d2997227053ab6ca76a2ac857

Modified Files
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src/backend/access/index/genam.c | 6 ++++--
src/backend/access/index/indexam.c | 17 +++++++++++------
2 files changed, 15 insertions(+), 8 deletions(-)