tiny change towards better error handling

[njasm]

This PR is a small initial refactorization for better error handling.

• included log and env_logger crates and added some console messages.
• uniformed functions and methods that returns Result<T, E> with a wide range of Es into anyhow::Error since we already have that crate in the app.
• refactor some unwrap() calls that could make the application panic (mainly on the gh api requests section) to better cope with eventual errors and not letting the app abort because of spurious errors.
• plus one or two minor changes (mainly cosmetics)

[pietroalbini]

Left some comments. In general I'd prefer to use an explicit Result<T, Error> instead of just Result<T>. It's a few more chars to write, but it's also more explicit.

[pietroalbini]

Thanks!