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/tests/block_verification.rs

@@ -160,11 +160,7 @@ async fn chain_segment_full_segment() {
         .into_block_error()
         .expect("should import chain segment");
 
-    harness
-        .chain
-        .recompute_head_at_current_slot()
-        .await
-        .expect("should run fork choice");
+    harness.chain.recompute_head_at_current_slot().await;
 
     assert_eq!(
         harness.head_block_root(),
@@ -194,11 +190,7 @@ async fn chain_segment_varying_chunk_size() {
                 .unwrap_or_else(|_| panic!("should import chain segment of len {}", chunk_size));
         }
 
-        harness
-            .chain
-            .recompute_head_at_current_slot()
-            .await
-            .expect("should run fork choice");
+        harness.chain.recompute_head_at_current_slot().await;
 
         assert_eq!(
             harness.head_block_root(),
@@ -729,11 +721,7 @@ async fn block_gossip_verification() {
     }
 
     // Recompute the head to ensure we cache the latest view of fork choice.
-    harness
-        .chain
-        .recompute_head_at_current_slot()
-        .await
-        .unwrap();
+    harness.chain.recompute_head_at_current_slot().await;
 
     /*
      * This test ensures that: