The union content store
- iterates through all the stores it has until the current store has the content.
- Or, it fails eventually if none of the stores have the content.
It does so by relying on the current store throwing a KeyError if it doesn't
have the content.
remotetreedatastore was throwing the MissingNodesError which means any
remaining stores after it would not even get a chance to look for the content.
This commit addresses the same.