ActiveResource.exists() should raise error instead of returning False for non 404 errors

[seaworn]

The exception clause here https://github.com/Shopify/pyactiveresource/blob/master/pyactiveresource/activeresource.py#L437 is too broad. For example, the method returns False for a 429 error due rate limiting which does not necessarily mean the resource does not exist. It should return False only for connection.ResourceNotFound otherwise raise the error