Need a logging convention

[thockin]

We should write down guidelines for when to use slog.Info* vs slog.Debug* vs.slog.Warn* vs. slog.Error*

I'm very biased, but I think Warn is pointless (but I like V levels, so ...).

Proposed:

• use Info for things you always want to see
• use Debug for things you want only when "verbose" is enabled
• don't use Warn
• use Error for things that require human attention

A "normal" log should not include Error lines.