Exception during normal operation in MirrorSourceTask causes the task to fail instead of shutting down gracefully

In MirrorSourceTask we are loading offsets for the topic partitions. At this point, while we are fetching the partitions, it is possible for the offset reader to be stopped by a parallel thread. Stopping the reader causes a CancellationException to be thrown, due to KAFKA-9051.

Currently this exception is not caught in MirrorSourceTask and so the exception propagates up and causes the task to go into FAILED state. We only need it to go to STOPPED state so that it would be restarted later.

This can be achieved by catching the exception and stopping the task directly.