Geocoder should raise exception if quote is exceeded

Re: https://github.com/tangentlabs/django-oscar-stores/blob/master/stores/services.py#L5

This function is a bit naive at the moment.  It needs to look more closely at the returned response and check for the various other error conditions that can occur.  

You'll need to a look at the Google maps docs to see the various error codes that can be returned.

We should raise an exception if an error occurs.  Make sure that the client code that calls this function catches and handles these exceptions gracefully.