Show dedicated message for tools with no entrypoints (#4694)
## Summary
Closes https://github.com/astral-sh/uv/issues/4688.
## Test Plan
```
❯ cargo run tool install ruff
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.14s
Running `target/debug/uv tool install ruff`
warning: `uv tool install` is experimental and may change without warning.
Resolved 1 package in 136ms
Installed 1 package in 3ms
+ ruff==0.5.0
No entrypoints to install for tool `ruff`
```
