`--project <path>` does not fail if `<path>` does not exist

### Summary

```
❯ uv run --project /tmp/does-not-exist python
Python 3.14.0 (main, Nov 19 2025, 23:04:28) [Clang 21.1.4 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 
```

At the very least, I think we should warn?

### Platform

n/a

### Version

n/a

### Python version

n/a