This method allows the query builder to check if a value is queryable and can be used to perform subqueries.

Those checks were performed in 4 different places in the query builder so the repetition is now extracted to a single method.

The valid values for performing subqueries are query builder instances, closures and strings.

If the value is not one of the above, an InvalidArgumentException was thrown without any message. I've added a sensible message, and also some `@throws` docblock tags to the affected methods.

I've also added some tests that confirm these methods can indeed throw exceptions when misused.