Improve the error message for logical replication of regular column to generated column.

Hi all,

Recently there was an issue reported by Kuroda-san on a different
thread [1]. I have created this thread to discuss the issue
separately.

Currently, the ERROR message for the replication of a regular column
on the publisher node to a generated column on the subscriber node
is:-
ERROR: logical replication target relation \"%s.%s\" is missing
replicated columns: %s
For example:-
test_pub=# CREATE TABLE t1(c1 int, c2 int GENERATED ALWAYS AS (c1 * 2) STORED);
test_pub=# CREATE PUBLICATION pub1 for table t1(c1, c2, c3);
test_pub=# INSERT INTO t1 VALUES (1);

test_sub=# CREATE TABLE t1(c1 int, c2 int GENERATED ALWAYS AS (c1 + 2)
STORED, c3 int GENERATED ALWAYS AS (c1 + 2) STORED);
test_sub=# CREATE SUBSCRIPTION sub1 CONNECTION '$publisher_connstr'
PUBLICATION pub1;
-> ERROR: logical replication target relation "t1" is missing
replicated column: "c2","c3"

The error message was misleading, as it failed to clarify that the
replication of a regular column on the publisher to the corresponding
generated column on the subscriber is not supported.
To avoid and solve the issue, we can update the ERROR message stating
that the replication of the generated column on the subscriber is not
supported. I have attached a patch for the same.

[1]: https://www.postgresql.org/message-id/TYCPR01MB5693AF061D62E55189490D2DF5562%40TYCPR01MB5693.jpnprd01.prod.outlook.com

Thanks and Regards,
Shubham Khanna.