Improve error handling for AssetServer::add_async (#13745)

# Objective

The method `AssetServer::add_async` (added in
https://github.com/bevyengine/bevy/pull/13700) requires a future that
returns an `AssetLoadError` error, which was a bit of an oversight on my
part, as that type of error only really makes sense in the context of
bevy's own asset loader -- returning it from user-defined futures isn't
very useful.

## Solution

Allow passing custom error types to `add_async`, which get cast into a
trait object matching the form of `AssetLoader::load`. If merged before
the next release this will not be a breaking change

crates/bevy_asset/src/server/mod.rs

@@ -718,9 +718,9 @@ impl AssetServer {
     ///
     /// After the asset has been fully loaded, it will show up in the relevant [`Assets`] storage.
     #[must_use = "not using the returned strong handle may result in the unexpected release of the asset"]
-    pub fn add_async<A: Asset>(
+    pub fn add_async<A: Asset, E: std::error::Error + Send + Sync + 'static>(
         &self,
-        future: impl Future<Output = Result<A, AssetLoadError>> + Send + 'static,
+        future: impl Future<Output = Result<A, E>> + Send + 'static,
     ) -> Handle<A> {
         let handle = self
             .data
@@ -741,12 +741,15 @@ impl AssetServer {
                             .unwrap();
                     }
                     Err(error) => {
+                        let error = AddAsyncError {
+                            error: Arc::new(error),
+                        };
                         error!("{error}");
                         event_sender
                             .send(InternalAssetEvent::Failed {
                                 id,
                                 path: Default::default(),
-                                error,
+                                error: AssetLoadError::AddAsyncError(error),
                             })
                             .unwrap();
                     }
@@ -1403,6 +1406,8 @@ pub enum AssetLoadError {
     CannotLoadIgnoredAsset { path: AssetPath<'static> },
     #[error(transparent)]
     AssetLoaderError(#[from] AssetLoaderError),
+    #[error(transparent)]
+    AddAsyncError(#[from] AddAsyncError),
     #[error("The file at '{}' does not contain the labeled asset '{}'; it contains the following {} assets: {}",
             base_path,
             label,
@@ -1441,6 +1446,22 @@ impl AssetLoaderError {
     }
 }
 
+#[derive(Error, Debug, Clone)]
+#[error("An error occurred while resolving an asset added by `add_async`: {error}")]
+pub struct AddAsyncError {
+    error: Arc<dyn std::error::Error + Send + Sync + 'static>,
+}
+
+impl PartialEq for AddAsyncError {
+    /// Equality comparison is not full (only through `TypeId`)
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        self.error.type_id() == other.error.type_id()
+    }
+}
+
+impl Eq for AddAsyncError {}
+
 /// An error that occurs when an [`AssetLoader`] is not registered for a given extension.
 #[derive(Error, Debug, Clone, PartialEq, Eq)]
 #[error("no `AssetLoader` found{}", format_missing_asset_ext(.extensions))]