Improve new hash partition bound check error messages

 Improve new hash partition bound check error messages
 
 For the error message "every hash partition modulus must be a factor
 of the next larger modulus", add a detail message that shows the
 particular numbers and existing partition involved.  Also comment the
 code more.
 
 Reviewed-by: Amit Langote <amitlangote09@gmail.com>
 Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
 Discussion: https://www.postgresql.org/message-id/flat/bb9d60b4-aadb-607a-1a9d-fdc3434dddcd%40enterprisedb.com