Tracking: error_generic_member_access

[jwuensche]

To propagate backtrace properly with thiserror we require two features to land in stable.

rust-lang/rust#99301
rust-lang/rust#96024

We encounter this issue as we start removing old error-chain errors and replacing them with thiserror impls. Once both are available we can write errors as such:

use thiserror::Error;

#[derive(Error, Debug)
pub enum CoolError {
    IoError {
        #[backtrace]
        source: std::io::Error,
    }
}

Which will automatically fetch errors from the provide member.