zabbix_sender outputs like this:

Info from server: "Processed 1 Failed 0 Total 1 Seconds spent 0.001593"
sent: 1; skipped: 0; total: 1

1) the only way to see unknown hosts is the "info from server" line, and this is hard to read. "processed: 1; Failed: 0; Total: 1" would be much clearer.
2) an exit value would be great, just "return (skipped+failed)" or so.... would help me not to parse output now (which will break once you fix the output)