Comment 2 for bug 1668954

The attached MP was attempting to address the error while the message is being formatted which it's already fixed by https://github.com/freyes/charm-helpers/commit/1117ba6d00c3ae1d54835f17e42cef106a416f4c

But the underlying problem of raising the Exception prematurely needs to be solved, the absence of api_version attribute in the relation may simply mean that keystone is not ready and polling for N seconds is needed before raising the error.