This is such a debugging timesink that I've added two things to bcachefs to address it.

- private error codes; error codes are unique, corresponding to a specific source location. They're mapped to standard error codes when we exit from the bcachefs module; in bcachefs they make for much more useful error messages.

- error_throw tracepoint: any time we throw an error, we invoke a standard tracepoint. Extremely useful for debugging wonky-but-not-broken behavior.