MSRV: Use core::error::Error instead of std::error::Error

[joncinque]

Problem

A lot of places in the SDK use the std feature to enable an implementation of std::error::Error, but this isn't needed because the Error trait was moved down to `core.

As mentioned at #12 (comment) though, using core::error::Error instead of std::error::Error requires an MSRV bump to 1.81.

Proposed Solution

Whenever we want to make an MSRV bump, let's remove the std::error::Error usage everywhere in favor of core::error::Error. Maybe we can include that whenever we bump the edition to 2024.