queue: Prevent an AttributeError rather than swallowing it.

When the RabbitMQ server disappears, we log errors like these:

```
Traceback (most recent call last):
  File "./zerver/lib/queue.py", line 114, in json_publish
    self.publish(queue_name, ujson.dumps(body))
  File "./zerver/lib/queue.py", line 108, in publish
    self.ensure_queue(queue_name, do_publish)
  File "./zerver/lib/queue.py", line 88, in ensure_queue
    if not self.connection.is_open:
AttributeError: 'NoneType' object has no attribute 'is_open'

During handling of the above exception, another exception occurred:
[... traceback of connection failure inside the retried self.publish()]
```

That's a type error -- a programming error, not an exceptional
condition from outside the program.  Fix the programming error.

Also move the retry out of the `except:` block, so that if it also
fails we don't get the exceptions stacked on each other.  This is a
new feature of Python 3 which is sometimes indispensable for
debugging, and which surfaced this nit in the logs (on Python 2 we'd
never see the AttributeError part), but in some cases it can cause a
lot of spew if care isn't taken.

zerver/lib/queue.py

@@ -85,7 +85,7 @@ class SimpleQueueClient:
         # type: (str, Callable[[], None]) -> None
         '''Ensure that a given queue has been declared, and then call
            the callback with no arguments.'''
-        if not self.connection.is_open:
+        if self.connection is None or not self.connection.is_open:
             self._connect()
 
         if queue_name not in self.queues:
@@ -112,10 +112,11 @@ class SimpleQueueClient:
         # Union because of zerver.middleware.write_log_line uses a str
         try:
             self.publish(queue_name, ujson.dumps(body))
-        except (AttributeError, pika.exceptions.AMQPConnectionError):
+            return
+        except pika.exceptions.AMQPConnectionError:
             self.log.warning("Failed to send to rabbitmq, trying to reconnect and send again")
-            self._reconnect()
 
+        self._reconnect()
         self.publish(queue_name, ujson.dumps(body))
 
     def register_consumer(self, queue_name, consumer):