More gracefully fail on `pyproject.toml` error

まだ誰も着手していません。

tried going through the "create environment..." flow with a `pyproject.toml` file but I had an error since my `pyproject.toml` file was incorrectly formatted. Would be good if the error message was better or it failed more gracefully

```
Resolved Python Environment /Users/eleanorboyd/python-envs-scenarios/existing-env-folder/.venv/bin/python
2025-05-14 09:19:39.308 [info] Running: uv pip install --python /Users/eleanorboyd/python-envs-scenarios/existing-env-folder/.venv/bin/python -e /Users/eleanorboyd/python-envs-scenarios/existing-env-folder
2025-05-14 09:19:39.378 [info] error: Failed to extract static metadata from `pyproject.toml`
Caused by: TOML parse error at line 6, column 8
|
6 | name = "existing env folder"
| ^^^^^^^^^^^^^^^^^^^^^
Not a valid package or extra name: "existing env folder". Names must start and end with a letter or digit and may only contain -, _, ., and alphanumeric characters.

```

「create environment...」フローと、ログに記録された `uv pip install --python ... -e ...` の呼び出しから始め、壊れた `pyproject.toml` の例を使って失敗を再現します。フローがパッケージメタデータのエラーをより明確に報告するか、生の失敗を露出させずに正常に終了すれば完了です。

索引モデルが issue の本文から書いたものです。