Consider ditching `flex-error` for `core::error::Error`

### Description

Rust 1.81 stabilized the `error_in_core` feature, allowing stable access to `core::error::Error`. This eliminates the original motivation for `flex-error`.

See also: https://github.com/informalsystems/flex-error/issues/8

### Definition of "done"

`flex-error` is no longer a dependency.
