Added ignoredErrorCodes to options for allow ignoring e.g. 401s

This is especially useful for hooking into Backbone. In my app, for example, I pop a login modal on 401. I don't need messenger to pop as well:

```javascript
$.globalMessenger().hookBackboneAjax({
  errorMessage: "Error contacting server. Please try again.",
  ignoredErrorCodes: [401]
});
```