Better error message for mismatched handle types

[JMS55]

I've seen a couple of users run into errors like these:

Requested handle of type TypeId { t: 201640482042465126479302603612250925020 } for asset 'ship.gltf' does not match actual asset type 'bevy_gltf::Gltf', which used loader 'bevy_gltf::loader::GltfLoader'

assertion left == right failed: The target AssetId<bevy_scene::scene::Scene>'s TypeId does not match the TypeId of this UntypedAssetId left: TypeId { t: 27553811827391490898130069402547565260 } right: TypeId { t: 189582940862555916391852589716973810489 }

What's happening is they load a whole GLTF file or scene and try to use it somewhere a single Handle<Mesh> is expected, instead of loading the mesh subasset of the GLTF file.

We should improve these error messages to ask users to double check what type of asset is expected.