Don't crash if dist/prebuilt folder is missing

e.g. I ran this:

```bash
LLM_USER_PATH='/tmp' llm 'Five names for a happy pet chipmunk'
```
And got this error:
```
  File "/opt/homebrew/Caskroom/miniconda/base/lib/python3.10/pathlib.py", line 1017, in iterdir
    for name in self._accessor.listdir(self):
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/mlc/dist/prebuilt'
```