Missing `std::error::Error` impl on `S7Error` enum

[uwe-nanninga]

The S7Error enum does not implement std::error::Error, making it incompatible with APIs that accept Box<dyn std::error::Error> or error handling crates such as anyhow.

Example

fn foo() -> Result<(), Box<dyn std::error::Error>> {
    Err(S7Error::ConnectionClosed)?;
    Ok(())
}

Actual behaviour

error[E0277]: `?` couldn't convert the error: `S7Error: std::error::Error` is not satisfied

Expected behaviour

Code compiles successfully.