sql/pgwire: Return expected ProtocolViolationError
sql/pgwire: Return expected ProtocolViolationError

This change returns the expected `ProtocolViolationError` Postgres error
code on unrecognized client message types.

glycerine/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,maxlang/cockroach,maxlang/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,glycerine/cockroach,veteranlu/cockroach,veteranlu/cockroach,paperstreet/cockroach,sunya123/cockroach,vaibhavarora/cockroach,maxlang/cockroach,petermattis/cockroach,cockroachdb/cockroach,veteranlu/cockroach,arjun4084346/cockroach,petermattis/cockroach,vivekmenezes/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,paperstreet/cockroach,sunya123/cockroach,maxlang/cockroach,petermattis/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,glycerine/cockroach,sunya123/cockroach,glycerine/cockroach,arjun4084346/cockroach,vaibhavarora/cockroach,sunya123/cockroach,veteranlu/cockroach,arjun4084346/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,petermattis/cockroach,sunya123/cockroach,glycerine/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,sunya123/cockroach,sunya123/cockroach,arjun4084346/cockroach,vaibhavarora/cockroach,petermattis/cockroach,veteranlu/cockroach,arjun4084346/cockroach,petermattis/cockroach,maxlang/cockroach,paperstreet/cockroach,cockroachdb/cockroach,maxlang/cockroach,maxlang/cockroach,cockroachdb/cockroach,paperstreet/cockroach,petermattis/cockroach,sunya123/cockroach,glycerine/cockroach,cockroachdb/cockroach,petermattis/cockroach,cockroachdb/cockroach,paperstreet/cockroach,maxlang/cockroach,veteranlu/cockroach,glycerine/cockroach,glycerine/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,arjun4084346/cockroach