Improper recovery from failed hydration

[tomsmalley]

There are cases which can lead to duplication of elements when we bail out of hydration. Seems like we are correctly creating and appending the new DOM, but not removing the old DOM properly. I have a feeling we are skipping those duplicated nodes before we mark hydration as having failed, so they end up remaining in the DOM. This is a consequence of trying to be too lenient about the DOM we accept - perhaps we should be stricter and avoid skipping potential extra nodes at all.

See https://github.com/obsidiansystems/obelisk/issues/428