AJAXError always passes default message back

#### What's this PR do?
The exception will always pass back a message even if the raise does not include one. A django application can set up AJAX_DEFAULT_ERROR_MESSAGE in settings.py to override this.

#### How should this be manually tested?
Create an exception that does not include a message. (It still must include a http status code)

Rather than doing this. We can just use https://github.com/exitio/django-ajax/blob/master/ajax/conf.py#L5 to setup the default. That way this setting will always exist.