transcode: Make sure we propagate not found err upstream

Otherwise the backoff retry loop wouldn't stop.

This was the only use of clients.GetFile that wasn't wrapping
the error properly.

transcode/transcode.go

@@ -538,7 +538,7 @@ func transcodeSegment(
 		defer cancel()
 		rc, err := clients.GetFile(ctx, transcodeRequest.RequestID, segment.Input.URL.String(), nil)
 		if err != nil {
-			return fmt.Errorf("failed to download source segment %q: %s", segment.Input, err)
+			return fmt.Errorf("failed to download source segment %q: %w", segment.Input, err)
 		}
 		defer rc.Close()