pgsql: Fix error message in check_partition_bounds_for_split_range()

Fix error message in check_partition_bounds_for_split_range()

Currently, the error message is produced by a system of complex substitutions
making it quite untranslatable and hard to read.  This commit splits this into
4 plain error messages suitable for translation.

Reported-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/20240408.152402.1485994009160660141.horikyota.ntt%40gmail.com
Reviewed-by: Pavel Borisov

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/96c7381c4ceee2ef7c0a7327beb71dec47cf855e

Modified Files
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src/backend/partitioning/partbounds.c | 70 ++++++++++++++++++++++++-----------
1 file changed, 49 insertions(+), 21 deletions(-)