allow access to the source error of AssetLoaderError and downcasting (#19471)

# Objective

I have a custom asset loader, and need access to the error it reports
when failing to load (e.g. through `AssetLoadFailedEvent { error:
AssetLoadError::AssetLoaderError(loader_error), .. }`). However
`AssetLoaderError` doesn't expose its `<core::error::Error>::source()`
(i.e. its `error` field. It only formats it when `Display`ed.

*I haven't searched for issues about it.*

## Solution

- Annotate `AssetLoaderError`'s `error` field with `#[source]`.
- Don't include the error when `AssetLoaderError` is `Display`ed (when
one prints an error's source stack like a backtrace, it would now be
dupplicated).
- (optional, included as a separated commit) Add a getter for the `&dyn
Error` stored in the `error` field (whithin an `Arc`). This is more
ergonomic than using `Error::source()` because it casts an `&Arc<dyn
Error>` into an `&dyn Error`, meaning one has to downcast it twice to
get the original error from the loader, including once where you have to
specify the correct type of the *private* `error` field. So downcasting
from `Error::source()` effectively rely on the internal implementation
of `AssetLoaderError`. The getter instead return the trait object
directly, which mean it will directly downcast to the expected loader
error type.

I didn't included a test that checks that double-downcasting
`<AssetLoaderError as Error>::source()` doesn't break user code that
would rely on the private field's type.

## Testing

- Downcasting the trait objects for both `source()` and the `error()`
getter work as described above.
- `cargo test -p bevy_asset --all-features` pass without errors.

---------

Co-authored-by: austreelis <git@swhaele.net>

crates/bevy_asset/src/server/mod.rs

@@ -1953,6 +1953,14 @@ impl AssetLoaderError {
     pub fn path(&self) -> &AssetPath<'static> {
         &self.path
     }
+
+    /// The error the loader reported when attempting to load the asset.
+    ///
+    /// If you know the type of the error the asset loader returned, you can use
+    /// [`BevyError::downcast_ref()`] to get it.
+    pub fn error(&self) -> &BevyError {
+        &self.error
+    }
 }
 
 /// An error that occurs while resolving an asset added by `add_async`.