Don't hard fail on sparse setup error

It seems this can fail in some casees, but proceed
with the download anyway.

server/download.go

@@ -216,9 +216,7 @@ func (b *blobDownload) run(ctx context.Context, requestURL *url.URL, opts *regis
 		return err
 	}
 	defer file.Close()
-	if err := setSparse(file); err != nil {
-		return err
-	}
+	setSparse(file)
 
 	_ = file.Truncate(b.Total)