mqtt.protocol.handler: Use exc_info to emit exception not %s

The former emits the stack trace, which greatly helps in identifying
_WHERE_ an exception is raised, not just WHAT the exception is.

amqtt/mqtt/protocol/handler.py

@@ -530,8 +530,8 @@ async def _send_packet(self, packet):
             await self.handle_connection_closed()
         except asyncio.CancelledError:
             raise
-        except Exception as e:
-            self.logger.warning("Unhandled exception: %s" % e)
+        except:
+            self.logger.warning("Unhandled exception", exc_info=True)
             raise
 
     async def mqtt_deliver_next_message(self):