cli: better error messaging around "interface/package" not found

Hi! I was following along with the [wasi-logging] proposal, but couldn't get `wit-bindgen` to generate markdown:

```shell
$ wit-bindgen rust wit/world.wit
Error: interface or world `logging` does not exist
--> wit/world.wit:4:12
|
4 | import logging;
|
```

After a little while I realized that somewhere along the line, I had started passing a full file path to wit-bindgen instead of a directory – `wit-bindgen markdown wit/world.wit` instead of `wit-bindgen markdown wit`.

Would you be amenable to adding a hint to the error output in the case where a package couldn't be found, but the input was a single file? (I'd be happy to make a PR!)

[wasi-logging]: https://github.com/WebAssembly/wasi-logging