Validate options on a joined query

[joelmukuthu]

Adding fields on a joined query for a count query causes an ambiguous error:

Group.count({ join: User.query.setOptions({ fields: ['id'] }) });
// rejects with:
// QueryError: Group: cannot join `User` with no primary or unique fields selected

And that error won't go away even if one selects a primary field on the parent Query:

// also fails:
Group.count({ join: User.query.setOptions({ fields: ['id'] }), fields: ['id'] });

In this case, it's a user error to add fields on a joined query, but the error message could be improved.