Reduce some EE and builder related ERRO logs to WARN (sigp#3966)

NA

Our `ERRO` stream has been rather noisy since the merge due to some unexpected behaviours of builders and EEs. Now that we've been running post-merge for a while, I think we can drop some of these `ERRO` to `WARN` so we're not "crying wolf".

The modified logs are:

I'm seeing this quite frequently on Geth nodes. They seem to timeout when they're busy and it rarely indicates a serious issue. We also have logging across block import, fork choice updating and payload production that raise `ERRO` or `CRIT` when the EE times out, so I think we're not at risk of silencing actual issues.

In sigp#3775 we reduced this log from `CRIT` to `ERRO` since it's common for builders to fail to reveal the block to the producer directly whilst still broadcasting it to the networ. I think it's worth dropping this to `WARN` since it's rarely interesting.

I elected to stay with `WARN` since I really do wish builders would fulfill their API promises by returning the block to us. Perhaps I'm just being pedantic here, I could be convinced otherwise.

It seems like builders and/or mev-boost struggle to handle heavy loads of validator registrations. I haven't observed issues with validators not actually being registered, but I see timeouts on these endpoints many times a day. It doesn't seem like this `ERRO` is worth it.

This means we failed to respond to a peer on the P2P network with a block they requested because of an error in the `execution_layer`. It's very common to see timeouts or incomplete responses on this endpoint whilst the EE is busy and I don't think it's important enough for an `ERRO`. As long as the peer count stays high, I don't think the user needs to be actively concerned about how we're responding to peers.

NA

beacon_node/execution_layer/src/engines.rs

@@ -364,7 +364,7 @@ impl Engine {
                 Ok(result)
             }
             Err(error) => {
-                error!(
+                warn!(
                     self.log,
                     "Execution engine call failed";
                     "error" => ?error,

beacon_node/http_api/src/lib.rs

@@ -2903,7 +2903,7 @@ pub fn serve<T: BeaconChainTypes>(
                     .await
                     .map(|resp| warp::reply::json(&resp))
                     .map_err(|e| {
-                        error!(
+                        warn!(
                             log,
                             "Relay error when registering validator(s)";
                             "num_registrations" => filtered_registration_data.len(),