Skip invalid tools in `uv tool list` (#5156)
## Summary
Makes the `tools()` return value include per-tool errors. This makes it
easy to skip (rather than failing) in `uv tool list`, _and_ improves `uv
tool uninstall` to remove those invalid tools, rather than leaving them
around. (We already had that behavior for `uv tool uninstall ruff` with
an invalid `ruff`, but `uv tool uninstall --all` just left them.)
Closes https://github.com/astral-sh/uv/issues/5151.
