asdf install should throw error on missing plugin

### Is your feature request related to a problem? Please describe

I am highly inexperienced with asdf, and ran `asdf install` with a `.tool-versions` nearby.
Yet, asdf didn't install anything. Turns out I had no plugins installed, so no tool could be downloaded.

My suggestion is that when running `asdf install`, asdf should at least give a warning on why a tool from `.tool-versions` was not installed.

Maybe I completely misunderstood the function of `asdf install` ....

### Describe the proposed solution

My suggestion would be a message like
```
"nodejs 22.15.1" was not installed because no plugin is available. Try "asdf plugin add nodejs"
"java openjdk-17" was not ...
```

### Describe similar `asdf` features and why they are not sufficient

no idea...

### Describe other workarounds you've considered

no idea

Start at the `asdf install` command and trace how entries from `.tool-versions` are handled when no plugin is installed. Done means the command clearly reports each skipped tool and suggests adding its missing plugin, with existing install behavior preserved.

Written by the indexing model from the issue text.