Capturing the original error code using the Logger Callback

[atrbgithub]

Hello, lets say a logger callback was implemented, something along the lines of the example here.

Is there a way to inspect the original error code, such as ERR__TRANSPORT: -195

Currently it only seems that the message is exposed to the logger, and not the code. Currently these are logged as errors, when perhaps you might only want to log these as a warn.

Currently the only way to do this seems to be by inspecting the log message string, rather than being able to directly inspect the error code.

It would be helpful if that could be exposed in the extra object.