[feature request] provide language-independent identifiers for errors

[DerekNonGeneric]

According to the Internationalization Best Practices for Spec Developers published by W3C Internationalization Working Group, we should be providing language-independent identifiers for errors.

It seems like the code property on each of our error instances would be the most appropriate place for this, but we are currently using constant-case identifiers (e.g., ERR_MISSING_OPTION like Node.js core) for this. Depending on how these errors are being checked at run-time, and if we want to be able to maintain interop with Node.js in the future, it may be worthwhile to use a different property for the language-independent identifier, which will most likely be composed strictly of digits.