pgsql: Improve error reporting for tuple-routing failures.

Improve error reporting for tuple-routing failures.

Currently, the whole row is shown without column names. Instead,
adopt a style similar to _bt_check_unique() in ExecFindPartition()
and show the failing key: (key1, ...) = (val1, ...).

Amit Langote, per a complaint from Simon Riggs. Reviewed by me;
I also adjusted the grammar in one of the comments.

Discussion: http://postgr.es/m/9f9dc7ae-14f0-4a25-5485-964d9bfc19bd@lab.ntt.co.jp

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/5a73e17317e91912b2755f7960d5bf31d374cf31

Modified Files
--------------
src/backend/access/index/genam.c | 4 ++
src/backend/catalog/partition.c | 30 ++++----
src/backend/executor/execMain.c | 132 ++++++++++++++++++++++++++++++-----
src/backend/utils/adt/ruleutils.c | 37 +++++++---
src/include/catalog/partition.h | 8 ++-
src/include/utils/ruleutils.h | 2 +
src/test/regress/expected/insert.out | 38 ++++++++--
src/test/regress/sql/insert.sql | 30 ++++++++
8 files changed, 231 insertions(+), 50 deletions(-)