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

@@ -16,26 +16,32 @@ 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"""
 
-    with open(results_file) as f:
-        data = f.read().strip()
-    pieces = data.split("|")
-
-    if not len(pieces) == 4:
+    try:
+        with open(results_file) as f:
+            data = f.read().strip()
+    except FileNotFoundError:
         state = "UNKNOWN"
         ret = 3
-        data = "Results file malformed"
+        data = "Results file is missing"
     else:
-        timestamp = int(pieces[0])
+        pieces = data.split("|")
 
-        time_diff = int(time.time() - timestamp)
-        if time_diff > max_time_diff:
-            ret = 3
+        if not len(pieces) == 4:
             state = "UNKNOWN"
-            data = "Results file is stale"
+            ret = 3
+            data = "Results file malformed"
         else:
-            ret = int(pieces[1])
-            state = pieces[2]
-            data = pieces[3]
+            timestamp = int(pieces[0])
+
+            time_diff = int(time.time() - timestamp)
+            if time_diff > max_time_diff:
+                ret = 3
+                state = "UNKNOWN"
+                data = "Results file is stale"
+            else:
+                ret = int(pieces[1])
+                state = pieces[2]
+                data = pieces[3]
 
     return (ret, f"{state}: {data}")