Introduce messaging interface between NIF and app #25
Description
At the moment, commands (e.g. connect port, shutdown) are sent to JACK asynchronously. When they succeed, messages are sent through Notifications. When they fail, the error is suppressed. This issue is to introduce two concepts:
- A way to notify ExJack when an error occurs in Rust during an async command
- A way to submit multiple commands of the same type