[ISSUE 80] Skipping generating data replication command when remote command returns error

[caigy]

Try to fix #80 :

1. Return error in broker_controller.exec() if remote command is failed.
2. Check if broker_controller.exec() returns error. If so, operator would NOT generating command to write the remote command results into specified files, which will write <nil> into subscriptionGroup.json or topics.json and cause startup failure of broker.