This adds `.code` attributes to errors returned by the op-layer, facilitating classifying OS errors and helping node-compat.
Similar to Node, these `.code` attributes are stringified names of unix ERRNOs, the mapping tables are generated by [tools/codegen_error_codes.js](https://gist.github.com/AaronO/dfa1106cc6c7e2a6ebe4dba9d5248858) and derived from libuv and rust's std internals
