Reword "Shards of relations in outer join queries must have 1-to-1 shard partitioning" error message

Follow the steps below:

``` SQL
CREATE TABLE test_1 (key int, dummy int);
CREATE TABLE test_2 (key int, dummy int);
SELECT master_create_distributed_table('test_1', 'key', 'hash');
SELECT master_create_distributed_table('test_2', 'key', 'hash');
SELECT master_create_worker_shards('test_1', 4, 1);
SELECT master_create_worker_shards('test_2', 4, 1);

SELECT * FROM test_1 as a LEFT JOIN test_2 as b ON (a.dummy = b.dummy);
```

I'd expect to see `cannot run outer joins with re-partition jobs`, but the actual message is: `Shards of relations in outer join queries must have 1-to-1 shard partitioning`, which is wrong since there is a 1-1 shard partitioning.