Prior to this patch:

```
root@:26257/> select * from pg_catalog.pg_class;
pq: no database specified
```

After:

```
root@:26257/> select * from pg_catalog.pg_class;
pq: cannot access virtual schema in anonymous database
HINT: verify that the current database is set
```

Release note (sql change): the error message produced upon accessing a
virtual schema with no database prefix (e.g. when `database` is not
set) is now clarified.