Make more noise when the EL is broken (#3986)

## Issue Addressed

Closes #3814, replaces #3818.

## Proposed Changes

* Add a WARN log for the case where we are attempting to sync chain segments but can't process them because they're building on an invalid parent. The most common case where we see this is when the execution node database is corrupt, causing sync to stall mysteriously (because we're currently logging the failure only at debug level).
* Additionally I've bumped up the logging for invalid execution payloads to `WARN`. This may result in some duplicate logs as we log errors from the `beacon_chain` and then again from the beacon processor. Invalid payloads and corrupt DBs _should_ be rare enough that this doesn't produce overwhelming log volume.

beacon_node/beacon_chain/src/block_verification.rs

@@ -280,10 +280,10 @@ pub enum BlockError<T: EthSpec> {
     ///
     /// ## Peer scoring
     ///
-    /// TODO(merge): reconsider how we score peers for this.
-    ///
-    /// The peer sent us an invalid block, but I'm not really sure how to score this in an
-    /// "optimistic" sync world.
+    /// The peer sent us an invalid block, we must penalise harshly.
+    /// If it's actually our fault (e.g. our execution node database is corrupt) we have bigger
+    /// problems to worry about than losing peers, and we're doing the network a favour by
+    /// disconnecting.
     ParentExecutionPayloadInvalid { parent_root: Hash256 },
 }