Ambiguous error tracker

Sticky issue to track all ambiguous/unhelpful errors, add more as comments

Please keep comments for sub-issues in this format:

> * ` for `
>
> ```sh-session
> $ wkg [...]
> Error:
>
> Caused by:
> 0:
> ```

### Errors

* interface not found for fetch

```sh-session
$ wkg fetch --config .wkg/config.toml
Error: failed to fetch dependencies for wit

Caused by:
0: failed to merge package from directory `wit`
1: interface not found in package
```

Reproduce the listed case with `wkg fetch --config .wkg/config.toml` and trace the source of the `interface not found in package` error chain. The issue names no file or test; done means making this fetch failure more specific and useful, then verifying the command's output.

Written by the indexing model from the issue text.