Add errors for missing error codes

[nemith]

There are additional error codes added to the server that we don't always handle ok. For example -6 is missing for UnimplementedException. See #49.

We just need to go through https://github.com/apache/zookeeper/blob/190a227aa9d4655ebfe6ba9f5c2da426da8c5d98/zookeeper-server/src/main/java/org/apache/zookeeper/KeeperException.java#L229 and make sure we have coverage. For v1 these need to be just regular errors but in v2 we should look at doing a ZKPathError or something like that that could add contextual information to the error (#32)