This pull request improves error message when extension loading goes
wrong.

Before:

```
2025-11-08T21:16:02+08:00 ERROR [extension_host::extension_host] failed to load arkts extension.toml

Caused by:
    No such file or directory (os error 2)
```

Now:

```
2025-11-08T22:57:00+08:00 ERROR [extension_host::extension_host] failed to load arkts extension.toml, "/Users/user_name_placeholder/Library/Application Support/Zed/extensions/installed/arkts/extension.toml"

Caused by:
    No such file or directory (os error 2)

```

Release Notes:

- N/A