Collector must close gracefully when rabbitmq is not available

Now it just panics
```
2021/04/17 13:35:37 Failed to connect to RabbitMQ (amqp://guest:guest@rabbitmq:5672/): dial tcp: lookup rabbitmq: no such host
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x13523bb]
```