Conditionally implement `core::error::Error` for all error types

[nicholasbishop]

In the same spirit as #587, we should implement the Error trait for all our other error types (e.g. these ones: https://docs.rs/uefi/latest/uefi/data_types/index.html#enums). This would be gated behind the unstable feature.

[phip1611]

We should be able to remove the unstable feature in 2 months or so. Since Rust 1.65, the trait is part of libcore in stable Rust.

rust-lang/rust@bf7611d

Correction, this is in the core library but not yet stable.

• https://doc.rust-lang.org/stable/src/core/error.rs.html#32-195
• Tracking Issue for Error in core rust-lang/rust#103765

[phip1611]

The change needs to be done at least for:

• unicode_collation::StrConversionError