[Chore]: Log startup errors

[matteojug]

Chore - Log startup errors

1. Description

The errors returned from main are not logged to the usual logs, eg if we fail to parse the config (missing some required fields):

sbtc/signer/src/main.rs

Line 81 in 67778db

let settings = Settings::new(args.config)?;

We should log whatever error we get from main before exiting.

2. Why It's Necessary

Without it, we just see no more logs, which is not really nice.