few places where this is used:
if(result.documents[0]['$err']...) { return callback(MongoError.create(result.documents[0]));
e.g. Server.prototype.command, executeSingleOperation ..

Possible fix:
/lib/connection/commands.js

Response.prototype.parse
...
// tell callers of the response body parsing that an error has occurred due to a unique index error (as one possible scenario) - if there is an error let the document be an error for the top level checking to work..
if (this.documents[i].writeErrors)