Abort Update if specified readConsistency fails

* Changed behavior for Update with readConsistency != ReadLocal
  If the read fails the update will now be aborted and replied
  with ReadFailure. Previously it continued with the local value.

* I think I was able to recollect the reason why I implemented it the other way. I had the ShoppingCart use case in mind and wanted to have an easy way to do an best effort ReadQuorum/WriteQuorum with fallback to local value if the ReadQuorum fails. I think the use case @hseeberger had in mind is more useful. We could support both, but I don't want to add more variants than really needed.

Fixes #50