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