nixos/tts: fix error messages read before text

with a config like this :

```
services.tts = {
  servers = {
    english = {
      enable = true;
      port = 5300;
      model = "tts_models/en/ljspeech/vits"
    };
  };
};
```

You the WAVs tts creates contain an error message which will be read to
you before the text you typed in will be read to you.
This patch fixes that.