Allow to passthrough errors

In ARR any error on the backend server is displayed as a 502, is there a way to display to the client the actual respond from the backend server.

the reason that I would like to be able to this, is because I have an app that checks if a file exist, so the app is expecting to see 404 or 200but because ARR will give a 502 instead of 404 then my app doesn't know if it was an error 500 or a 404, any ideas how to just allow the status code to pass through??

thanks

J