[audit] Handle errors when fetching storage keys from Unstablebackend (#1440)

* Handle errors when fetching storage keys from Unstablebackend

* cargo fmt

subxt/src/backend/unstable/storage_items.rs

@@ -150,6 +150,11 @@ impl<T: Config> Stream for StorageItems<T> {
                     self.buffered_responses = items.items;
                     continue;
                 }
+                FollowEvent::OperationError(err) if err.operation_id == *self.operation_id => {
+                    // Something went wrong obtaining storage items; mark as done and return the error.
+                    self.done = true;
+                    return Poll::Ready(Some(Err(Error::Other(err.error))));
+                }
                 _ => {
                     // We don't care about this event; wait for the next.
                     continue;