nagios: Don’t crash on missing cron file.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

scripts/nagios/cron_file_helper.py

@@ -11,8 +11,14 @@ def nagios_from_file(results_file: str) -> Tuple[int, str]:
     This file is created by various nagios checking cron jobs such as
     check-rabbitmq-queues and check-rabbitmq-consumers"""
 
+    try:
         with open(results_file) as f:
             data = f.read().strip()
+    except FileNotFoundError:
+        state = 'UNKNOWN'
+        ret = 3
+        data = "Results file is missing"
+    else:
         pieces = data.split('|')
 
         if not len(pieces) == 4: