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

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

Read the assets migration guide section on removed anyhow and compare it with the Bevy Asset v2 section linked in the issue. Identify the missing custom asset loader error-type guidance, reconcile the two sections, and confirm that the migration instructions match the main section.

Written by the indexing model from the issue text.