Transfer impl of release_attach to new home

A bunch of code has been dropped because it's no longer necessary -- yay for better data builder usage and real error handling!

src/lib.rs

@@ -15,6 +15,7 @@ pub enum Error {
     Placeholder, // TODO: Enumerate error modes
     WrappedReqwestErr(reqwest::Error),
     MissingAuthToken,
+    NoSuchFile, // for release attachment 'file exists' pre-check.
 }
 
 impl From<reqwest::Error> for crate::Error {