return type-erased Errors in AssetLoader::load

[rlidwka]

Fixes #10350

ping @bushrat011899, @Roryl-c (I'm not actually using custom assets, so this needs to be reviewed by someone who does)

Objective

Want to allow people to use anyhow as the return error value of AssetLoader.

Solution

ErasedAssetLoader::load already returns type-erased errors. Why not return the same errors in AssetLoader::load?

Also, I aliased bevy::asset::Error to Box<dyn Error + Send + Sync + 'static> to make changes more backward compatible in the future + easier to type. This was suggested earlier in #5359.

Changelog

AssetLoader::load now returns type-erased Errors.

Migration Guide

Remove an associated type Error from AssetLoader and AssetSaver. Return error value is now Box<dyn Error> to allow for unsized user types.

[Runi-c]

I tried this out and was met with a very confusing type error.
I re-read the PR and saw that I had to change the return type to:
BoxedFuture<'a, Result<Self::Asset, BoxedError>>
instead of using anyhow::Error as the error type. This is fine, but I would guess others might be similarly confused when they migrate.

I tried #10493 as well and that one was trivial for me as I was able to set the associated type directly to anyhow::Error as I was instructed by the original migration guide. So that might be a preferred fix for this case assuming it doesn't have other problems.

[alice-i-cecile]

Closing in favor of #10493. Thank you!