Improve INSERT .. ON CONFLICT error message.

Peter Geoghegan, reviewed by me.

src/backend/executor/execIndexing.c

@@ -535,8 +535,8 @@ ExecCheckIndexConstraints(TupleTableSlot *slot,
 
 		if (!indexRelation->rd_index->indimmediate)
 			ereport(ERROR,
-					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("ON CONFLICT does not support deferred unique constraints/exclusion constraints as arbiters"),
+					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+					 errmsg("ON CONFLICT does not support deferrable unique constraints/exclusion constraints as arbiters"),
 					 errtableconstraint(heapRelation,
 								   RelationGetRelationName(indexRelation))));
 

src/test/regress/output/constraints.source

@@ -643,7 +643,7 @@ INSERT INTO deferred_excl VALUES(1); -- fail
 ERROR:  conflicting key value violates exclusion constraint "deferred_excl_con"
 DETAIL:  Key (f1)=(1) conflicts with existing key (f1)=(1).
 INSERT INTO deferred_excl VALUES(1) ON CONFLICT ON CONSTRAINT deferred_excl_con DO NOTHING; -- fail
-ERROR:  ON CONFLICT does not support deferred unique constraints/exclusion constraints as arbiters
+ERROR:  ON CONFLICT does not support deferrable unique constraints/exclusion constraints as arbiters
 BEGIN;
 INSERT INTO deferred_excl VALUES(2); -- no fail here
 COMMIT; -- should fail here