POC: Add new `Error` type for the public API

[benjamin-nw]

This PR is a PoC to show an implementation with an Error and a Result type for the tokio-modbus crate.

The goal of the PoC is to test if updating the std::io::Error into a crate::error::Error can be an effective solution to better handle Exception returned by a modbus server.

Working examples:

• [ ] rtu-client-sync.rs
• [ ] rtu-client.rs
• [ ] rtu-server-address.rs
• [ ] rtu-server.rs
• [ ] tcp-client-custom-fn.rs
• [ ] tcp-client-sync.rs
• [x] tcp-client.rs
• [x] tcp-server.rs
• [ ] tls-client.rs
• [ ] tls-server.rs

Please, feel free to review this PR anytime.