Error message inconsistency

As noted by a PostgreSQL user to me, error messages for NOT NULL
constraints are inconsistent - they do not mention the relation name in the
message, as all other variants of this message do. e.g.

postgres=# create table nn (id integer not null);
CREATE TABLE
postgres=# insert into nn values (NULL);
ERROR: null value in column "id" violates not-null constraint
DETAIL: Failing row contains (null).

postgres=# create table nn2 (id integer check (id is not null));
CREATE TABLE
postgres=# insert into nn2 values (NULL);
ERROR: new row for relation "nn2" violates check constraint "nn2_id_check"
DETAIL: Failing row contains (null).

I propose the attached patch as a fix, changing the wording (of the first
case) to
ERROR: null value in column "id" for relation "nn" violates not-null
constraint

It causes breakage in multiple tests, which is easy to fix once/if we agree
to change.

Thanks

--
Simon Riggs http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
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