gs: Don't drop error when finishing upload

The error returned when finishing the upload of an object was dropped.
This could cause silent upload failures and thus data loss in certain
cases. When a MD5 hash for the uploaded blob is specified, a wrong
hash/damaged upload would return its error via the Close() whose error
was dropped.

changelog/unreleased/pull-3249

@@ -0,0 +1,7 @@
+Bugfix: Better error handling for gs backend
+
+The gs backend did not notice when the last steep of completing a file upload
+failed. Under rare circumstance, this might be able to cause missing files in
+the backup repository. This has been fixed.
+
+https://github.com/restic/restic/pull/3249

internal/backend/gs/gs.go

@@ -235,7 +235,10 @@ func (be *Backend) Save(ctx context.Context, h restic.Handle, rd restic.RewindRe
 	w := be.bucket.Object(objName).NewWriter(ctx)
 	w.ChunkSize = 0
 	wbytes, err := io.Copy(w, rd)
-	w.Close()
+	cerr := w.Close()
+	if err == nil {
+		err = cerr
+	}
 
 	be.sem.ReleaseToken()