check_cron_file: Don't crash on missing cron file.

This is 5050fb19f6, but for `check_cron_file`, which was introduced
in 91da4bd59b.

puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_cron_file

@@ -16,8 +16,14 @@ def nagios_from_file(results_file: str, max_time_diff: int = 60 * 2) -> "Tuple[i
     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: