add logging of serverStatus errors

The current code does not check if db.serverStatus() returns an error (e.g. insufficient privileges). If that is the case, no message is logged anywhere, so user has no clue why metrics are not being posted. I propose to log a warning with the error message.

checks.d/mongo.py

@@ -195,6 +195,9 @@ def check(self, instance):
                 self.log.error("Mongo: cannot connect with config %s" % server)
 
         status = db["$cmd"].find_one({"serverStatus": 1})
+        if status['ok'] == 0:
+            self.log.warn(status['errmsg'].__str__())
+
         status['stats'] = db.command('dbstats')
 
         # Handle replica data, if any