cql3: make column names readable for invalid delete statements
cql3: make column names readable for invalid delete statements

This commit makes the column names from an invalid delete statement human readable.
Before that, they were printed in their hex representation, which is not convenient
for debugging.

Before:
  InvalidRequest:
    Error from server: code=2200 [Invalid query]
    message="Invalid where clause contains non PRIMARY KEY columns: 76616c"
After:
  InvalidRequest:
    Error from server: code=2200 [Invalid query]
    message="Invalid where clause contains non PRIMARY KEY columns: val"
Message-Id: <d77ffad60488aa7783102bbab5cd3cd607e993c1@scylladb.com>

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