Improve error message when trying to load glb file without label

## Bevy version and features

- main (cbf989c9dab5ce8152ab9407e0d4cb42b0138003)

## What you did

Attempting to load a glb without a label:

```rust
commands.spawn(SceneRoot(asset_server.load("out.glb")));
```

## What went wrong

Results in this error message:

```
ERROR bevy_asset::server: Could not find an asset loader matching: Loader Name: None; Asset Type: None; Extension: None; Path: Some("out.glb");
```

which makes it sound glb files aren't supported. We ran into this issue on Discord https://discord.com/channels/691052431525675048/692572690833473578/1410977714302746854 and we confusingly thought it was something to do with the newly introduced web assets.

The fix is easy, just add `#Scene0` to the url or use [GltfAssetLabel](https://docs.rs/bevy/latest/bevy/gltf/prelude/enum.GltfAssetLabel.html) which would be useful as a suggestion in the error message.