I like very much something I saw in the Dropbox API: All application errors are returned with a 409 code, and then you are sure to have a properly json formatt

I like very much something I saw in the Dropbox API: All application errors are returned with a 409 code, and then you are sure to have a properly json formatted error in the body that can will have an error code more relevant for your case. So that could be NOT_FOUND in such a case.