Indicate no quorum checkpoint (#2737)

### Description

Currently, when the validator is not signing, there is just a unhelpful
"Could not fetch metadata" log line. This indicates that no quorum could
be found at the debug level as well as adding the checkpoint syncer
information

### Backward compatibility

Yes

### Testing

Manual

rust/hyperlane-base/src/types/multisig.rs

@@ -355,6 +355,7 @@ impl MultisigCheckpointSyncer {
                 continue;
             }
         }
+        debug!("No quorum checkpoint found for message");
         Ok(None)
     }
 }