Don't return errors when fork choice fails (#3370)

## Issue Addressed

NA

## Proposed Changes

There are scenarios where the only viable head will have an invalid execution payload, in this scenario the `get_head` function on `proto_array` will return an error. We must recover from this scenario by importing blocks from the network.

This PR stops `BeaconChain::recompute_head` from returning an error so that we can't accidentally start down-scoring peers or aborting block import just because the current head has an invalid payload.

## Reviewer Notes

The following changes are included:

1. Allow `fork_choice.get_head` to fail gracefully in `BeaconChain::process_block` when trying to update the `early_attester_cache`; simply don't add the block to the cache rather than aborting the entire process.
1. Don't return an error from `BeaconChain::recompute_head_at_current_slot` and `BeaconChain::recompute_head` to defensively prevent calling functions from aborting any process just because the fork choice function failed to run.
    - This should have practically no effect, since most callers were still continuing if recomputing the head failed.
    - The outlier is that the API will return 200 rather than a 500 when fork choice fails.
1. Add the `ProtoArrayForkChoice::set_all_blocks_to_optimistic` function to recover from the scenario where we've rebooted and the persisted fork choice has an invalid head.

beacon_node/beacon_chain/src/state_advance_timer.rs

@@ -220,14 +220,7 @@ async fn state_advance_timer<T: BeaconChainTypes>(
                     return;
                 }
 
-                if let Err(e) = beacon_chain.recompute_head_at_slot(next_slot).await {
-                    warn!(
-                        log,
-                        "Error updating fork choice for next slot";
-                        "error" => ?e,
-                        "slot" => next_slot,
-                    );
-                }
+                beacon_chain.recompute_head_at_slot(next_slot).await;
 
                 // Use a blocking task to avoid blocking the core executor whilst waiting for locks
                 // in `ForkChoiceSignalTx`.