Add error handling to iterators (#1243)

* Add error handling to iterators

* Review feedback

* Leverage itertools::process_results() in few places

beacon_node/store/src/memory_store.rs

@@ -149,7 +149,7 @@ impl<E: EthSpec> Store<E> for MemoryStore<E> {
         end_state: BeaconState<E>,
         end_block_root: Hash256,
         _: &ChainSpec,
-    ) -> Self::ForwardsBlockRootsIterator {
+    ) -> Result<Self::ForwardsBlockRootsIterator, Error> {
         SimpleForwardsBlockRootsIterator::new(store, start_slot, end_state, end_block_root)
     }