Mark DynamicPluginLoadError internal error types as source

[BD103]

Objective

• thiserror is used to derive the error type on bevy_dynamic_plugin's DynamicPluginLoadError.
• It is an enum where each variant wraps a libloading error type.
• thiserror supports marking this internal error types as #[source] so it can automatically fill out the Error::source method.
• This allows other error handling libraries to get more information about the error than what Bevy by default provides. It increases interoperability between libraries.

Solution

• Mark the internal libloading::Error of DynamicPluginLoadError with #[source].

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Changelog

• Implemented the Error::source method for DynamicPluginLoadError.

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Here is the output from cargo-expand before and after the change.

// Before
impl Error for DynamicPluginLoadError {}

// After
impl Error for DynamicPluginLoadError {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        use thiserror::__private::AsDynError as _;

        match self {
            DynamicPluginLoadError::Library { 0: source, .. } => {
                Some(source.as_dyn_error())
            }
            DynamicPluginLoadError::Plugin { 0: source, .. } => {
                Some(source.as_dyn_error())
            }
        }
    }
}