SQL parser produces confusing error message when referenced table unused

The SQL

``` sql
select name from aaa, bbb where aaa.id = 7;
```

produces the confusing error message [Cannot read property 'rhs' of undefined](http://ql.io/console?s=select%20name%20from%20aaa,%20bbb%20where%20aaa.id%20=%207;).  (Because JOIN condition not satisfied?)
