Improve error message for replication of generated columns.

Improve error message for replication of generated columns.

Currently, logical replication produces a generic error message when
targeting a subscriber-side table column that is either missing or
generated. The error message can be misleading for generated columns.

This  introduces a specific error message to clarify the issue when
generated columns are involved.

Author: Shubham Khanna
Reviewed-by: Peter Smith, Vignesh C, Amit Kapila
Discussion: https://postgr.es/m/CAHv8RjJBvYtqU7OAofBizOmQOK2Q8h+w9v2_cQWxT_gO7er3Aw@mail.gmail.com