Adjust error message

We usually don't use "namespace" in user-facing error messages.  Also,
in master this was replaced by another error message referring to
"temporary objects", so we might as well use that here to avoid
introducing too many variants.

Discussion: https://www.postgresql.org/message-id/bbd3f8d9-e3d5-e5aa-4305-7f0121c3fa94@2ndquadrant.com

src/backend/access/transam/xact.c

@@ -2306,7 +2306,7 @@ PrepareTransaction(void)
 	if ((MyXactFlags & XACT_FLAGS_ACCESSEDTEMPNAMESPACE))
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-				 errmsg("cannot PREPARE a transaction that has operated on temporary namespace")));
+				 errmsg("cannot PREPARE a transaction that has operated on temporary objects")));
 
 	/*
 	 * Likewise, don't allow PREPARE after pg_export_snapshot.  This could be

src/test/modules/test_extensions/expected/test_extensions.out

@@ -148,7 +148,7 @@ SELECT create_extension_with_temp_schema();
 (1 row)
 
 PREPARE TRANSACTION 'twophase_extension';
-ERROR:  cannot PREPARE a transaction that has operated on temporary namespace
+ERROR:  cannot PREPARE a transaction that has operated on temporary objects
 -- Clean up
 DROP TABLE test_ext4_tab;
 DROP FUNCTION create_extension_with_temp_schema();

src/test/regress/expected/temp.out

@@ -310,19 +310,19 @@ begin;
 create function pg_temp.twophase_func() returns text as
   $$ select '2pc_func'::text $$ language sql;
 prepare transaction 'twophase_func';
-ERROR:  cannot PREPARE a transaction that has operated on temporary namespace
+ERROR:  cannot PREPARE a transaction that has operated on temporary objects
 -- Function drop
 create function pg_temp.twophase_func() returns text as
   $$ select '2pc_func'::text $$ language sql;
 begin;
 drop function pg_temp.twophase_func();
 prepare transaction 'twophase_func';
-ERROR:  cannot PREPARE a transaction that has operated on temporary namespace
+ERROR:  cannot PREPARE a transaction that has operated on temporary objects
 -- Operator creation
 begin;
 create operator pg_temp.@@ (leftarg = int4, rightarg = int4, procedure = int4mi);
 prepare transaction 'twophase_operator';
-ERROR:  cannot PREPARE a transaction that has operated on temporary namespace
+ERROR:  cannot PREPARE a transaction that has operated on temporary objects
 -- These generate errors about temporary tables.
 begin;
 create type pg_temp.twophase_type as (a int);
@@ -374,4 +374,4 @@ SELECT current_schema() ~ 'pg_temp' AS is_temp_schema;
 (1 row)
 
 PREPARE TRANSACTION 'twophase_search';
-ERROR:  cannot PREPARE a transaction that has operated on temporary namespace
+ERROR:  cannot PREPARE a transaction that has operated on temporary objects