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

src/backend/access/index/genam.c

@@ -168,6 +168,10 @@ IndexScanEnd(IndexScanDesc scan)
  * The passed-in values/nulls arrays are the "raw" input to the index AM,
  * e.g. results of FormIndexDatum --- this is not necessarily what is stored
  * in the index, but it's what the user perceives to be stored.
+ *
+ * Note: if you change anything here, check whether
+ * ExecBuildSlotPartitionKeyDescription() in execMain.c needs a similar
+ * change.
  */
 char *
 BuildIndexValueDescription(Relation indexRelation,