Return error if getBlobs not supported (#6911)

N/A


  Previously, we were returning an empty vec of Nones if get_blobs was not supported in the EL. This results in confusing logging where we try to process the empty list of blobs and log a bunch of Unexpected errors. See
```
Feb 03 17:32:12.383 DEBG Fetching blobs from the EL, num_expected_blobs: 6, block_root: 0x7326fe2dc1cb9036c9de7a07a662c86a339085597849016eadf061b70b7815ba, service: fetch_engine_blobs, service: beacon, module: beac
on_chain::fetch_blobs:84
Feb 03 17:32:12.384 DEBG Processing engine blobs, num_fetched_blobs: 0, block_root: 0x7326fe2dc1cb9036c9de7a07a662c86a339085597849016eadf061b70b7815ba, service: fetch_engine_blobs, service: beacon, module: beacon_c
hain::fetch_blobs:197
Feb 03 17:32:12.384 ERRO Error fetching or processing blobs from EL, block_root: 0x7326fe2dc1cb9036c9de7a07a662c86a339085597849016eadf061b70b7815ba, error: BlobProcessingError(AvailabilityCheck(Unexpected)), module
: network::network_beacon_processor:1011
```

The error we should be getting is that getBlobs is not supported, this PR adds a new error variant and returns that.

beacon_node/execution_layer/src/lib.rs

@@ -158,6 +158,7 @@ pub enum Error {
     },
     ZeroLengthTransaction,
     PayloadBodiesByRangeNotSupported,
+    GetBlobsNotSupported,
     InvalidJWTSecret(String),
     InvalidForkForPayload,
     InvalidPayloadBody(String),
@@ -1871,7 +1872,7 @@ impl<E: EthSpec> ExecutionLayer<E> {
                 .map_err(Box::new)
                 .map_err(Error::EngineError)
         } else {
-            Ok(vec![None; query.len()])
+            Err(Error::GetBlobsNotSupported)
         }
     }