Mock (REST) service, as a (mostly) passthrough

I'd like to create a Mock service that mostly acts as a pass-through to a real service that we use, but be able to quickly make one particular request fail in a particular way, so that I can address a particular case we've seen.

Anybody have advice or example script on the simplest way to simply pass an incoming request & response through a mock operation?

(Similar desire for a different use case, from this StackOverflow question, BTW: https://stackoverflow.com/questions/36041434/soapui-create-mock-service-with-pass-through-behavior-for-selected-methods )