clean up unwraps and error messages

[cfal]

there's quite a few places where we could be handling errors more gracefully, eg

$ ./shoes
No config specified, assuming loading from file config.shoes.yaml
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { kind: Io(Os { code: 2, kind: NotFound, message: "No such file or directory" }), paths: ["/home/jkl/shoes-async/config.shoes.yaml"] }', src/main.rs:71:14
stack backtrace:
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[18o]

do you accept PR?