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/http_api/src/lib.rs

@@ -1059,10 +1059,7 @@ pub fn serve<T: BeaconChainTypes>(
 
                         // Update the head since it's likely this block will become the new
                         // head.
-                        chain
-                            .recompute_head_at_current_slot()
-                            .await
-                            .map_err(warp_utils::reject::beacon_chain_error)?;
+                        chain.recompute_head_at_current_slot().await;
 
                         // Perform some logging to inform users if their blocks are being produced
                         // late.
@@ -1186,10 +1183,7 @@ pub fn serve<T: BeaconChainTypes>(
                         Ok(_) => {
                             // Update the head since it's likely this block will become the new
                             // head.
-                            chain
-                                .recompute_head_at_current_slot()
-                                .await
-                                .map_err(warp_utils::reject::beacon_chain_error)?;
+                            chain.recompute_head_at_current_slot().await;
 
                             Ok(warp::reply::json(&()))
                         }