internal/download: don't discard dst.Close error

When `io.Copy` succeeds but the buffered `Close` fails (e.g. disk full on `Flush`), the error was swallowed and verification reported a misleading hash mismatch instead of the real I/O failure. Keep the `Close` error when `io.Copy` didn't already produce one.

internal/download/download.go

@@ -210,7 +210,9 @@ func (db *ChecksumDB) DownloadFile(url, dstPath string) error {
 		dst = newDownloadWriter(fd, resp.ContentLength)
 	}
 	_, err = io.Copy(dst, resp.Body)
-	dst.Close()
+	if closeErr := dst.Close(); err == nil {
+		err = closeErr
+	}
 	if err != nil {
 		os.Remove(tmpfile)
 		return err