more specific exception in ActiveResource

Hi all, I created a patch wich allows to a nested resource raise an
ActiveResource::MissingPrefixParam exception instead of
ActiveResource::ResourceNotFound when we call a
method without prefix_params, IMO prefix_params should be required on
nested resource methods that uses element_path or collection_path.
The ticket with the patch and tests:

Any comments and feedback would be awesome, If you're interested, please test
and +1.