Best HTTP Response Code for a Restful Api which makes calls to other Web Services when a failure occurs

So i'm designing a Restful Api that makes calls to other web services aggregates the result and return back to the client. If connection of any of the other web services fails for any reason, what is the best thing to return?

Right now am returning an 500 - Internal Server error to the client but I would like to return more details to the client on what made the request to fail. Would it be redundant to return a 500 http response code with a response body containing a message detailing where the error actually occurred or to just return a 503 - Service Unavailable http response code?