pgsql: Make dblink try harder to form useful error messages

Make dblink try harder to form useful error messages

When libpq encounters a connection-level error, e.g. runs out of memory
while forming a result, there will be no error associated with PGresult,
but a message will be placed into PGconn's error buffer. postgres_fdw
takes care to use the PGconn error message when PGresult does not have
one, but dblink has been negligent in that regard. Modify dblink to mirror
what postgres_fdw has been doing.

Back-patch to all supported branches.

Author: Joe Conway
Reviewed-By: Tom Lane
Discussion: https://postgr.es/m/02fa2d90-2efd-00bc-fefc-c23c00eb671e%40joeconway.com

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/76943f54a743e3d7875a6772fec89bb57a8f2118

Modified Files
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contrib/dblink/dblink.c | 31 ++++++++++++++++++++++---------
1 file changed, 22 insertions(+), 9 deletions(-)