Message style improvement

Backpatch the part of edee0c621d that applies to a90bdd7a44, which
was also backpatched.  That way, the message is consistent in all
branches.

src/backend/commands/tablecmds.c

@@ -10052,7 +10052,7 @@ CloneFkReferencing(List **wqueue, Relation parentRel, Relation partRel)
 		if (fk->confrelid == RelationGetRelid(partRel))
 			ereport(ERROR,
 					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("can't attach table \"%s\" as a partition which is referenced by foreign key \"%s\"",
+					 errmsg("cannot attach table \"%s\" as a partition because it is referenced by foreign key \"%s\"",
 							RelationGetRelationName(partRel),
 							get_constraint_name(fk->conoid))));
 

src/test/regress/expected/foreign_key.out

@@ -1901,7 +1901,7 @@ ALTER TABLE fk_partitioned_fk ATTACH PARTITION fk_partitioned_fk_2
 CREATE TABLE fk_partitioned_pk_6 (a int PRIMARY KEY);
 CREATE TABLE fk_partitioned_fk_6 (a int REFERENCES fk_partitioned_pk_6) PARTITION BY LIST (a);
 ALTER TABLE fk_partitioned_fk_6 ATTACH PARTITION fk_partitioned_pk_6 FOR VALUES IN (1);
-ERROR:  can't attach table "fk_partitioned_pk_6" as a partition which is referenced by foreign key "fk_partitioned_fk_6_a_fkey"
+ERROR:  cannot attach table "fk_partitioned_pk_6" as a partition because it is referenced by foreign key "fk_partitioned_fk_6_a_fkey"
 DROP TABLE fk_partitioned_pk_6, fk_partitioned_fk_6;
 -- This case is similar to above, but the referenced relation is one level
 -- lower in the hierarchy.  This one fails in a different way as the above,