pgsql: Replace over-optimistic Assert in partitioning code with a runti

Replace over-optimistic Assert in partitioning code with a runtime test.

get_partition_parent felt that it could simply Assert that systable_getnext
found a tuple. This is unlike any other caller of that function, and it's
unsafe IMO --- in fact, the reason I noticed it was that the Assert failed.
(OK, I was working with known-inconsistent catalog contents, but I wasn't
expecting the DB to fall over quite that violently. The behavior in a
non-assert-enabled build wouldn't be very nice, either.) Fix it to do what
other callers do, namely an actual runtime-test-and-elog.

Also, standardize the wording of elog messages that are complaining about
unexpected failure of systable_getnext. 90% of them say "could not find
tuple for <object>", so make the remainder do likewise. Many of the
holdouts were using the phrasing "cache lookup failed", which is outright
misleading since no catcache search is involved.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e7941a976688f0f5d13a5227ed4f3efe0718db9d

Modified Files
--------------
contrib/sepgsql/database.c | 2 +-
contrib/sepgsql/proc.c | 4 ++--
contrib/sepgsql/relation.c | 8 ++++----
contrib/sepgsql/schema.c | 2 +-
src/backend/catalog/aclchk.c | 4 ++--
src/backend/catalog/objectaddress.c | 2 +-
src/backend/catalog/partition.c | 3 ++-
src/backend/commands/extension.c | 9 +++++----
src/backend/utils/adt/ruleutils.c | 2 +-
9 files changed, 19 insertions(+), 17 deletions(-)