Strict match of errors in backfill sync

[dapplion]

Issue Addressed

Same spirit as

• Stricter match of BlockError in lookup sync #6321

Handling errors in sync with a fallback match is not great. In the case of backfill sync it's very easy to fix! =D

(tangent) Noting a chat with @pawanjay176 in #6321 BeaconChainError actually includes errors that are not internal. In this case it was some of the variants of HistoricalBlockError.. I'll review BeaconChainError to double check there's no other hidden scorable variants.

Proposed Changes

Turns out that its processing returns almost only variants of HistoricalBlockError, so I changed import_historical_block_batch to return HistoricalBlockError instead of BeaconChainError.

All logic is the same, just made the match on network beacon processor cleaner a bit cleaner.