Fix error message for models sub-package

[lpiwowar]

Minor comment (which applies everywhere you call RuntimeError:
Using self.__class__.__name__ can provide a more specific class name if the error somehow originates from a subclass context related to _api_response not being set (though unlikely with current structure). Or simply ModelProvider if the intent is to always refer to the base.

Originally posted by @EmilienM in #154 (comment)