confusing messages during network discovery

ZBXNEXT-3581 introduced mandatory header validation. this has resulted in pretty confusing messages from network discovery.

Steps to reproduce:
create a network discovery that discovers a zabbix agent which does not allow connections from this server

Result:
at loglevel 3, log gets :

Message from <IP> is missing header. Message ignored.

Expected:
no messages printed at loglevel 3. at loglevel 4, instead of such a confusing message, server/proxy could log the familiar "agent closed connection, permission issue..." message (current behaviour at loglevel 4 not tested)