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.

consensus/proto_array/src/proto_array.rs

@@ -980,7 +980,7 @@ impl ProtoArray {
 /// Returns `None` if there is an overflow or underflow when calculating the score.
 ///
 /// https://github.com/ethereum/consensus-specs/blob/dev/specs/phase0/fork-choice.md#get_latest_attesting_balance
-fn calculate_proposer_boost<E: EthSpec>(
+pub fn calculate_proposer_boost<E: EthSpec>(
     validator_balances: &[u64],
     proposer_score_boost: u64,
 ) -> Option<u64> {