writeConcernError is a general field that can be attached to any CRUD response (and maybe other commands' responses too?), but it looks like it doesn't appear in any IDL file other than the file in which the WriteConcernError struct is defined. I would've expected it to be in either generic_argument.idl or in write_ops.idl. Does it need to be added here?

If it does, we should also clean up existing usages. For example in the bulkWrite project we added this field to the bulk write IDL file.