[COMMITTERS] pgsql: Improve dblink error message when remote does not provide it

Improve dblink error message when remote does not provide it

When dblink or postgres_fdw detects an error on the remote side of the
connection, it will try to construct a local error message as best it
can using libpq's PQresultErrorField(). When no primary message is
available, it was bailing out with an unhelpful "unknown error". Make
that message better and more style guide compliant. Per discussion
on hackers.

Backpatch to 9.2 except postgres_fdw which didn't exist before 9.3.

Discussion: https://postgr.es/m/19872.1482338965%40sss.pgh.pa.us

Branch
------
REL9_6_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/88673c9d3bc72004367e3317175cb873584405bf

Modified Files
--------------
contrib/dblink/dblink.c           | 2 +-
contrib/postgres_fdw/connection.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)