fix(index): export MongoNetworkError

Author: mbroadst

index.js

@@ -9,6 +9,7 @@ const connect = require('./lib/mongo_client').connect;
 
 // Expose error class
 connect.MongoError = core.MongoError;
+connect.MongoNetworkError = core.MongoNetworkError;
 
 // Actual driver classes exported
 connect.Admin = require('./lib/admin');