Remove logrus in favour of log/slog

[pkqk]

This lets us remove a dependency and use the standard library.

The API is a bit less direct than the logrus or even log packages but the message parts can be hidden in only a few places.

There is no formatting options of the log calls or a Fatal option so I've had to put explicit os.Exit(1) calls in the few locations.

I've also cleaned up some of the error messages to make them have consistent tenses and not use the word error in the error message.