fixes #151 removed column vis from public API, cahced col vis in impl, v...

...alidated col vis in impl
I was punting on this, it currently throws some guava exception wrapping an Accumulo exception.   We do need to something here, users may want to react to this specific case.   The entire API lacks a coherent strategy for exceptions.   What do you think about opening an issue for addressing exceptions in the API, and calling this case out in the issue?   I like the Zookeeper model for exceptions, where many things extend ZookeeperException.