Reply error code along with spqr router internal error message

[reshke]

we can define method just alike https://github.com/pg-sharding/spqr/blob/master/pkg/client/client.go#L31 to reply PostgreSQL proto error by code, not by message.

func (cl *PsqlClient) ReplyErrMsgByCode(errcode string) error {
        clerrmsg := getErrMsgByCode(errcode)
	for _, msg := range []pgproto3.BackendMessage{
		&pgproto3.ErrorResponse{
			Message:  clerrmsg,
			Severity: "ERROR",
                        Code: errcode
		},
		&pgproto3.ReadyForQuery{
			TxStatus: byte(txstatus.TXIDLE),
		},
	} {
		if err := cl.Send(msg); err != nil {
			return err
		}
	}

	return nil
}

https://github.com/postgres/postgres/blob/master/src/backend/utils/errcodes.txt#L482-L486
we can use

SPQRD for failed to match any datashard
SPQRE for skip executing this query, wait for next
SPQRC for ComplexQuery
SPQRF for FailedToMatch
SPQRS for SkipColumn
SPQRM for ShardingKeysMissing
SPQRX for CrossShardQueryUnsupported

[CBists]

SPQRR for Routing error
SPQRU for Unexpected error

[Denchick]

The feature was merged in #408