BUG #13865: PQresultStatus returns PGRES_COPY_OUT instead of PGRES_FATAL_ERROR for certain bad COPY statement

The following bug has been logged on the website:

Bug reference:      13865
Logged by:          Septian Hari
Email address:      septian.hari@gmail.com
PostgreSQL version: 9.4.5
Operating system:   ubuntu 14.04
Description:

We notice that for certain bad COPY statement, PQresultStatus will return
PGRES_COPY_OUT instead of PGRES_FATAL_ERROR, and subsequently hide the error
when using ruby pg gem.

Steps to reproduce:

    $ psql postgres
    psql (9.5.0, server 9.4.5)
    Type "help" for help.

    postgres=# create table xxx (properties json);
    CREATE TABLE
    postgres=# insert into xxx values ('{"a": "1\u000023"}');
    INSERT 0 1
    postgres=# select properties->'a' from xxx;
    ERROR:  unsupported Unicode escape sequence
    DETAIL:  \u0000 cannot be converted to text.
    CONTEXT:  JSON data, line 1: {"a":...

    $ pry
    [1] pry(main)> require 'pg'
    => true
    [2] pry(main)> conn = PG.connect(dbname: 'postgres')
    => #<PG::Connection:0x007fbac577ee08>
    [3] pry(main)> conn.exec "copy (select properties->'a' from xxx) to
stdout"
    => #<PG::Result:0x007fbac5855cf0 status=PGRES_COPY_OUT ntuples=0
nfields=1 cmd_tuples=0>
    [4] pry(main)> conn.get_copy_data
    => nil

    $ PSYCOPG_DEBUG=1 ipython
    ...
    In [5]: cur.copy_expert("COPY (SELECT properties->'a' FROM xxx) TO
STDOUT", sys.stdout)
    [4480] curs_execute: pg connection at 0x2eb9bb0 OK
    [4480] pq_begin_locked: pgconn = 0x2eb9bb0, autocommit = 0, status = 1
    [4480] pq_execute_command_locked: pgconn = 0x2eb9bb0, query = BEGIN
    [4480] pq_execute: executing SYNC query: pgconn = 0x2eb9bb0
    [4480]     COPY (SELECT properties->'a' FROM xxx) TO STDOUT
    [4480] pq_execute: entering synchronous DBAPI compatibility mode
    [4480] pq_fetch: pgstatus = PGRES_COPY_OUT
    [4480] pq_fetch: data from a COPY TO (no tuples)
    [4480] psycopg_is_text_file: importing io.TextIOBase
    [4480] _read_rowcount: PQcmdTuples =
    [4480] pq_raise: PQresultErrorMessage: err=ERROR:  unsupported Unicode
escape sequence
    DETAIL:  \u0000 cannot be converted to text.
    CONTEXT:  JSON data, line 1: {"a":...

    [4480] pq_raise: err2=unsupported Unicode escape sequence
    DETAIL:  \u0000 cannot be converted to text.
    CONTEXT:  JSON data, line 1: {"a":...

    [4480] pq_fetch: fetching done; check for critical errors

---------------------------------------------------------------------------
    DataError                                 Traceback (most recent call
last)
    <ipython-input-5-65d68a7d89bd> in <module>()
    ----> 1 cur.copy_expert("COPY (SELECT properties->'a' FROM xxx) TO
STDOUT", sys.stdout)

    DataError: unsupported Unicode escape sequence
    DETAIL:  \u0000 cannot be converted to text.
    CONTEXT:  JSON data, line 1: {"a":..