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]
```

Start by tracing the RabbitMQ connection setup and error path that produces the reported log and nil-pointer panic. Run the collector with RabbitMQ unavailable; done means it handles the connection failure and exits or shuts down gracefully without panicking.

Written by the indexing model from the issue text.