SourceAlignment announceCombinedWatermark period task maybe lost

When we use sourcealignment，we also found there is another problem that announceCombinedWatermark may throw a exception (like  "subtask 25 is not ready yet to receive events" , this subtask maybe under failover), which will lead the period task not running any more (ThreadPoolExecutor will not schedule the period task if it throw a exception)

I think we should increase the robustness of announceCombinedWatermark function to avoid it throw any exception (if send fail, just wait next send) (code see https://github.com/apache/flink/blob/master/flink-runtime/src/main/java/org/apache/flink/runtime/source/coordinator/SourceCoordinator.java#L199 )