Exit with failure when an error occured while connecting to mongod.

This avoids the exception being caught and ignored, letting the process hang in an unusable state.

This is particularly important when starting mongod, the cube collector and evaluator via [supervisord](supervisord.org).  If the process does not exit with failure, supervisord will never restart the collector and evaluator and the connection to mongod will never happen.

And, in general, there is no point in keeping the collector and evaluator running if there is no connection to mongod.

Thanks!