errors and logging improvements

[flokli]

make common::Error a proper error

This allows using it in eyre and chaining it with other errors.

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rust: improve logging and error formatting

Use the `tracing` crate to show logs, and honor `RUST_LOG` env var to
change log-level. Make some of the debugging statements debug! calls,
and fields rather than interpolated in the message.

Use the `.context(&'static str')?` pattern to annotate some info about
where things go wrong.`