Make AssetLoader/Saver Error type bounds compatible with anyhow

anyhow::Error does not implement std::error::Error but converting to boxed
std::error::Error is possible. At the same time this bound should work fine
with custom error types like ones generated by thiserror.
Note: after this change Bevy 0.12 migration guide claim that anyhow::Error
works in this context becomes true.
Fixes #10350

crates/bevy_asset/src/loader.rs

@@ -28,7 +28,7 @@ pub trait AssetLoader: Send + Sync + 'static {
     /// The settings type used by this [`AssetLoader`].
     type Settings: Settings + Default + Serialize + for<'a> Deserialize<'a>;
     /// The type of [error](`std::error::Error`) which could be encountered by this loader.
-    type Error: std::error::Error + Send + Sync + 'static;
+    type Error: Into<Box<dyn std::error::Error + Send + Sync + 'static>>;
     /// Asynchronously loads [`AssetLoader::Asset`] (and any other labeled assets) from the bytes provided by [`Reader`].
     fn load<'a>(
         &'a self,
@@ -90,7 +90,9 @@ where
                 .expect("Loader settings should exist")
                 .downcast_ref::<L::Settings>()
                 .expect("AssetLoader settings should match the loader type");
-            let asset = <L as AssetLoader>::load(self, reader, settings, &mut load_context).await?;
+            let asset = <L as AssetLoader>::load(self, reader, settings, &mut load_context)
+                .await
+                .map_err(|error| error.into())?;
             Ok(load_context.finish(asset, Some(meta)).into())
         })
     }

crates/bevy_asset/src/processor/process.rs

@@ -138,7 +138,7 @@ impl<Loader: AssetLoader, Saver: AssetSaver<Asset = Loader::Asset>> Process
                 .saver
                 .save(writer, saved_asset, &settings.saver_settings)
                 .await
-                .map_err(|error| ProcessError::AssetSaveError(Box::new(error)))?;
+                .map_err(|error| ProcessError::AssetSaveError(error.into()))?;
             Ok(output_settings)
         })
     }

crates/bevy_asset/src/saver.rs

@@ -14,7 +14,7 @@ pub trait AssetSaver: Send + Sync + 'static {
     /// The type of [`AssetLoader`] used to load this [`Asset`]
     type OutputLoader: AssetLoader;
     /// The type of [error](`std::error::Error`) which could be encountered by this saver.
-    type Error: std::error::Error + Send + Sync + 'static;
+    type Error: Into<Box<dyn std::error::Error + Send + Sync + 'static>>;
 
     /// Saves the given runtime [`Asset`] by writing it to a byte format using `writer`. The passed in `settings` can influence how the
     /// `asset` is saved.  
@@ -53,7 +53,9 @@ impl<S: AssetSaver> ErasedAssetSaver for S {
                 .downcast_ref::<S::Settings>()
                 .expect("AssetLoader settings should match the loader type");
             let saved_asset = SavedAsset::<S::Asset>::from_loaded(asset).unwrap();
-            self.save(writer, saved_asset, settings).await?;
+            if let Err(err) = self.save(writer, saved_asset, settings).await {
+                return Err(err.into());
+            }
             Ok(())
         })
     }