Sometimes during a `Db.createIndex` operation, the server will respond with a valid error to indicate that `createIndex` operation has failed.

Testcase: https://github.com/suprememoocow/node-mongodb-native-create-index-error/blob/master/test.js

For example, in the test-case where the server responds with:

```
code: 85
message: Index with pattern: ... already exists with different options
```

Instead of returning this information to the server, the server falls-back to attempting an insert into the `system.indexes` collection.

If the key for the index contains any period characters (which are valid for a the key, but not as a document for insertion into a collection) the insert will fail, leading to a situation where the user is presented with a cryptic error message instead of the original (understandable error).

So, instead of getting an error like the one I presented above, the user will receive one like:
```
Error: key a.one must not contain '.'
at Error (native)
at serializeInto (/../node-mongodb-native-create-index-error/node_modules/mongodb/node_modules/mongodb-core/node_modules/bson/lib/bson/parser/serializer.js:731:19)
at serializeObject (/../node-mongodb-native-create-index-error/node_modules/mongodb/node_modules/mongodb-core/node_modules/bson/lib/bson/parser/serializer.js:294:18)
at serializeInto (/../node-mongodb-native-create-index-error/node_modules/mongodb/node_modules/mongodb-core/node_modules/bson/lib/bson/parser/serializer.js:754:17)
at serializeObject (/../node-mongodb-native-create-index-error/node_modules/mongodb/node_modules/mongodb-core/node_modules/bson/lib/bson/parser/serializer.js:294:18)
at serializeInto (/../node-mongodb-native-create-index-error/node_modules/mongodb/node_modules/mongodb-core/node_modules/bson/lib/bson/parser/serializer.js:596:17)
at serializeObject (/../node-mongodb-native-create-index-error/node_modules/mongodb/node_modules/mongodb-core/node_modules/bson/lib/bson/parser/serializer.js:294:18)
at serializeInto (/../node-mongodb-native-create-index-error/node_modules/mongodb/node_modules/mongodb-core/node_modules/bson/lib/bson/parser/serializer.js:754:17)
at serialize (/../node-mongodb-native-create-index-error/node_modules/mongodb/node_modules/mongodb-core/node_modules/bson/lib/bson/bson.js:49:27)
at Query.toBin (/../node-mongodb-native-create-index-error/node_modules/mongodb/node_modules/mongodb-core/lib/connection/commands.js:143:25)
```

While this example may seem fairly contrived, it can easily happen in a more natural manner, especially when using mongoose (as we are). I would have saved a fair deal of time had I been able to access the original error instead of the incorrect one.