I found that duplicate key error (11000) raises a Mongo::Error::OperationFailure. The error object has `code` defined (= 11000) however it does not have `code_name` defined.

It appears code_names are mapped at the driver layer. It would be useful if the Mongo Ruby Driver could:

1. Include code name mappings for all error codes which are defined in the MongoDB server specification
2. Keep the mapping up-to-date when there are new major server version releases.

(There maybe other similarly mapped fields as well; I haven't looked in detail.)