0.12 migration: Custom asset loader code is missing the associated error type

[rparrett]

These changes came after the "assets migration guide" was written: https://bevyengine.org/learn/migration-guides/0-11-to-0-12/#removed-anyhow

We should reconcile that section with the content of the main assets v2 section: https://bevyengine.org/learn/migration-guides/0-11-to-0-12/#bevy-asset-v2

[aleksa2808]

I support this, it was confusing seeing the difference between the Assets V2 migration guide and the 0.12 examples. The Assets V2 section doesn't mention the new Error type that needs to be defined when implementing AssetLoaders, as well as the removal of anyhow in favour of thiserror.