more useful error message (#9014)

substrate/client/finality-grandpa/src/import.rs

@@ -646,9 +646,10 @@ where
 		initial_sync: bool,
 	) -> Result<(), ConsensusError> {
 		if justification.0 != GRANDPA_ENGINE_ID {
-			return Err(ConsensusError::ClientImport(
-				"GRANDPA can only import GRANDPA Justifications.".into(),
-			));
+			return Err(ConsensusError::ClientImport(format!(
+				"Expected GRANDPA Justification, got {}.",
+				String::from_utf8_lossy(&justification.0)
+			)));
 		}
 
 		let justification = GrandpaJustification::decode_and_verify_finalizes(