Fix zip export

It was broken since 67587450e47fb0d9bd2753edc2a04c5b9f93951c;

* If zip command was not found, subprocess.Popen() will raise an
  exception and the fallback path will not be reached. The old code
  worked because os.system() will just return a non-zero return status.
* If zip command was found, the call will also fail because --out does
  not do what I probably thought it does!