return memcached errors on failed datastore operation

When (for example) an insert fails at the KV level, N1QL returns:

_time=2018-03-20T07:30:13.991-07:00 _level=ERROR _msg=Failed to perform <ud>insert</ud> on key <ud>4.89899</ud> for Keyspace STOCK

note the lack of a KV error description.
We can do better than that...