SQL/JSON: Improve some error messages

SQL/JSON: Improve some error messages

This improves some error messages emitted by SQL/JSON query functions
by mentioning column name when available, such as when they are
invoked as part of evaluating JSON_TABLE() columns.  To do so, a new
field column_name is added to both JsonFuncExpr and JsonExpr that is
only populated when creating those nodes for transformed JSON_TABLE()
columns.

While at it, relevant error messages are reworded for clarity.

Reported-by: Jian He <jian.universality@gmail.com>
Suggested-by: Jian He <jian.universality@gmail.com>
Discussion: https://postgr.es/m/CACJufxG_e0QLCgaELrr2ZNz7AxPeGCNKAORe3fHtFCQLsH4J4Q@mail.gmail.com