Implement Error.code

mysql and mysql implement Error.code, a String that indicate variable name.
Normally connector must provide error number, connector must not have any need to know error variable name, but since provided, some framework now begin to rely on those code :
example : https://github.com/sequelize/sequelize/blob/3fedaa431667cabd51b524409a32415b830b0398/lib/dialects/mysql/connection-manager.js#L137

The only way to provide compatibility is to provide those additional bad information.