Improve logging & error output

[TheJolman]

Why

Logging and error handling is currently kinda all over the place in both the API and CLI. We're using a mix of both fmt and log for logging info and errors, and only sometimes sending errors to stderr with Fprintf.

How

1. Decide on a strategy for both binaries (they don't have to use the same one) and document it
2. Consistently apply pattern/strategy to codebase
3. Make sure errors/warnings are being sent to stderr and regular output that use should always see is being sent to stdout
4. Log level should be configurable

API

We can continue using log, but there are more powerful logging libraries that have easily configurable log levels (e.g., INFO, DEBUG, WARN, FATAL).
Options:

• Go's builtin structured logger, log/slog: https://pkg.go.dev/log/slog
• Charm has a prettier option, charmbracelet/log: https://github.com/charmbracelet/log

These are just suggestions. You can deviate from them but have a reason for doing so.

CLI

Cobra comes with error functions that we should almost def be using: https://pkg.go.dev/github.com/spf13/cobra#CheckErr