Warn that loading asset using a `PathBuf` does not handle labels

## What problem does this solve or what need does it fill?

Currently trying to load an asset passing a `PathBuf` with a label tries to load an asset with the label as part of the extension.

## What solution would you like?

Add a warning that `PathBuf` does not support loading labeled assets

## Additional context

doing `asset_server.load(PathBuf::from("a.test#Test"))` gives a `Path not found` error because the path that the `AssetReader` receives is `a.test#Test` instead of `a.test`