Removed anyhow

[bushrat011899]

Objective

• Fixes Reduce/remove the use of anyhow in the engine #8140

Solution

• Added Explicit Error Typing for AssetLoader and AssetSaver, which were the last instances of anyhow in use across Bevy.

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Changelog

• Added an associated type Error to AssetLoader and AssetSaver for use with the load and save methods respectively.
• Changed ErasedAssetLoader and ErasedAssetSaver load and save methods to use Box<dyn Error + Send + Sync + 'static> to allow for arbitrary Error types from the non-erased trait variants. Note the strict requirements match the pre-existing requirements around anyhow::Error.

Migration Guide

• anyhow is no longer exported by bevy_asset; Add it to your own project (if required).
• AssetLoader and AssetSaver have an associated type Error; Define an appropriate error type (e.g., using thiserror), or use a pre-made error type (e.g., anyhow::Error). Note that using anyhow::Error is a drop-in replacement.
• AssetLoaderError has been removed; Define a new error type, or use an alternative (e.g., anyhow::Error)
• All the first-party AssetLoader's and AssetSaver's now return relevant (and narrow) error types instead of a single ambiguous type; Match over the specific error type, or encapsulate (Box<dyn>, thiserror, anyhow, etc.)

Notes

A simpler PR to resolve this issue would simply define a Bevy Error type defined as Box<dyn std::error::Error + Send + Sync + 'static>, but I think this type of error handling should be discouraged when possible. Since only 2 traits required the use of anyhow, it isn't a substantive body of work to solidify these error types, and remove anyhow entirely. End users are still encouraged to use anyhow if that is their preferred error handling style. Arguably, adding the Error associated type gives more freedom to end-users to decide whether they want more or less explicit error handling (anyhow vs thiserror).

As an aside, I didn't perform any testing on Android or WASM. CI passed locally, but there may be mistakes for those platforms I missed.

[alice-i-cecile]

This is much nicer. Thank you!

[james7132]

Thanks for diving into this. This has been bothering me since I started using Bevy. It's notably more verbose, but also much more explicit. Much more Rust-like this way. Glad to see it's finally being addressed.

[bushrat011899]

Looks like I missed one docstring in the bevy_gltf package (which previously didn't require documentation). I've merged main back into this PR and resolved that issue.

[cart]

These AssetLoadError::CannotLoadDependency errors don't make sense to me:

1. We're dropping the actual loader error contents, making the error message effectively useless.
2. Not all asset loads are dependencies, but we're returning that error for every asset load failure.

[cart]

We're also no longer including meta deserialization failures / throwing them in the generic CannotLoadDependency bucket.

[cart]

I'm just going to fix this in Multiple Asset Sources as it touches this code.

[Runi-c]

AssetLoader and AssetSaver have an associated type Error; Define an appropriate error type (e.g., using thiserror), or use a pre-made error type (e.g., anyhow::Error). Note that using anyhow::Error is a drop-in replacement.

Not sure if a comment is the right place to put this, but this section of this PR's migration guide is incorrect. anyhow::Error does not implement Error, which is required by these associated types - so it does not work as a drop-in replacement.