cql3: throw proper request exception for INSERT JSON
cql3: throw proper request exception for INSERT JSON

JSON code is amended in order to return proper
"Missing mandatory PRIMARY KEY part" message instead of generic
"Attempt to access value of a disengaged optional object".

Fixes #3665
Message-Id: <b83b2b6255cb246b4cad15451f856976d9885d26@scylladb.com>

avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla