Update Memcached unsupported operations message to include Ephemeral bucket types

Unsupported MemcachedBucket operations include the message "This method is only supported on Couchbase Bucket (persistent) types.". However, this is not accurate anymore because of Ephemeral buckets. Not all messages require updating, eg View operations and methods that use PersistTo parameters are unsupported on Ephemeralbucekts too.