SQL/JSON: Fix error-handling of some JsonBehavior expressions

SQL/JSON: Fix error-handling of some JsonBehavior expressions

To ensure that the errors of executing a JsonBehavior expression that
is coerced in the parser are caught instead of being thrown directly,
pass ErrorSaveContext to ExecInitExprRec() when initializing it.
Also, add a EEOP_JSONEXPR_COERCION_FINISH step to handle the errors
that are caught that way.

Discussion: https://postgr.es/m/CACJufxEo4sUjKCYtda0_qt9tazqqKPmF1cqhW9KBOUeJFqQd2g@mail.gmail.com
Backpatch-through: 17