implement core::error::Error

[johann-cm]

Implement the core::error::Error trait to better integrate the atat Error enums into the rust error system.

My use case: be able to use the #[from] attribute to derive an error enum for my application:

#[derive(Debug, Error)]
pub enum MyApplicationError {
    #[error("AT error: {0}")]
    ATError(#[from] atat::Error),
    ...
}

[MathiasKoch]

This seems reasonable. Thank you for the contribution, and sorry for the long review time.

Could you rebase to re-trigger the CI?