pgsql: Refactor/reword some error messages to avoid duplicates

Refactor/reword some error messages to avoid duplicates Also, remove brackets around "EMPTY [ ARRAY ]". An error message is not the place to state that a keyword is optional. Backpatch to 17. Branch -- REL_17_STABLE Details --- https://git.postgresql.org/pg/commitdiff/899f39ea25f2ee63a

pgsql: Refactor/reword some error messages to avoid duplicates

Refactor/reword some error messages to avoid duplicates Also, remove brackets around "EMPTY [ ARRAY ]". An error message is not the place to state that a keyword is optional. Backpatch to 17. Branch -- master Details --- https://git.postgresql.org/pg/commitdiff/2bb969f3998489e5dc4fe9f2