The CLI installed no logger, so every log::warn! and log::error! was
dropped: a plugin that failed to load just vanished from
engine.clap_plugins with no reason. A small logger now writes warnings
and errors to standard error as `warning: <message>` and
`error: <message>`, like the CLI's own errors. The level is fixed:
RUST_LOG stays a desktop app variable.
Fixes#1304.