Fix: Add HTTP status code error handling to prevent download hangs (#495)

When download requests receive HTTP error responses (like 429, 503, etc.), the code was previously waiting indefinitely for chunks that would never arrive.
This adds `response.error_for_status_ref()?` which immediately fails with a clear
error message when the server returns an error status code.

Fixes an issue where some modpack downloads would hang at 0 B/s indefinitely.

Author: TheSpareTire177

libium/src/upgrade/mod.rs

@@ -284,6 +284,7 @@ impl DownloadData {
         );
 
         let mut response = client.get(url).send().await?;
+        response.error_for_status_ref()?;
 
         while let Some(chunk) = response.chunk().await? {
             temp_file.write_all(&chunk)?;