[assets] properly set LoadState with invalid asset extension (#2318)

# Objective

- Currently, when calling any of the `AssetServer`'s `load` functions, if the extension does not exist for the given path, the returned handle's load state is always `LoadState::NotLoaded`. 
- This is due to the `load_async` function early returning without properly creating a `SourceInfo` for the requested asset.
- Fixes #2261

## Solution
- Add the `SourceInfo` prior to checking for valid extension loaders. And set the `LoadState` to `Failed` if the according loader does not exist.

crates/bevy_asset/src/asset_server.rs

@@ -236,7 +236,6 @@ impl AssetServer {
         asset_path: AssetPath<'_>,
         force: bool,
     ) -> Result<AssetPathId, AssetServerError> {
-        let asset_loader = self.get_path_asset_loader(asset_path.path())?;
         let asset_path_id: AssetPathId = asset_path.get_id();
 
         // load metadata and update source info. this is done in a scope to ensure we release the
@@ -280,6 +279,15 @@ impl AssetServer {
             source_info.load_state = LoadState::Failed;
         };
 
+        // get the according asset loader
+        let asset_loader = match self.get_path_asset_loader(asset_path.path()) {
+            Ok(loader) => loader,
+            Err(err) => {
+                set_asset_failed();
+                return Err(err);
+            }
+        };
+
         // load the asset bytes
         let bytes = match self.server.asset_io.load_path(asset_path.path()).await {
             Ok(bytes) => bytes,
@@ -708,7 +716,7 @@ mod test {
             _ => false,
         });
 
-        assert_eq!(asset_server.get_load_state(handle), LoadState::NotLoaded);
+        assert_eq!(asset_server.get_load_state(handle), LoadState::Failed);
     }
 
     #[test]