[SPARK-2670] FetchFailedException should be thrown when local fetch has failed

I thought wrong. Exactly, in current usage of BlockFetcherIterator, next() is not invoked after FetchFailedException has been thrown.
I wonder it's a little bit problem that we can invoke next() after FetchFailedException even if there are no such usages in current implementation.
I think it's better to prohibit invoking next() after FetchFailedException to clearly express the correct usage of the method.