Rebalancing hangs after failure to request segments

When the segments request fails, StateConsumerImpl.onTaskCompletion() doesn't do anything.

Usually the failure is because the source node left, and then a topology update should happen soon. But sometimes the exception is not caused by the node leaving, and the InboundTransferTask remains in a limbo state because there is no topology update to follow the exception.

We should let the coordinator know that something went wrong and the rebalance may need to be restarted, and we should also allow the user to force-cancel the rebalance and force-start a new one.