Improve handling of missing interpreters during discovery (#4218)
Cherry-picked from https://github.com/astral-sh/uv/pull/4214
The first commit gets us some context on an IO error during queries:
Previously:
```
failed to canonicalize path `[VENV]/bin/python3`
Caused by: No such file or directory (os error 2)
```
Now:
```
Failed to query Python interpreter
Caused by: failed to canonicalize path `[VENV]/bin/python3`
Caused by: No such file or directory (os error 2)
```
but really we shouldn't attempt to query a missing interpreter during
discovery anyway, so we improve handling of that too.
