[ML] Improve log message for model snapshot update

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/job/process/autodetect/JobModelSnapshotUpgrader.java

@@ -196,15 +196,18 @@ private void deleteOlderSnapshotDoc(SearchResponse searchResponse, Consumer<Exce
                     );
                 }
             }, e -> {
-                logger.warn(
-                    () -> format(
+                logger.warn(() -> {
+                    String baseMessage = format(
                         "[%s] [%s] failed to delete old snapshot [%s] result document",
                         jobId,
                         snapshotId,
                         ModelSizeStats.RESULT_TYPE_FIELD.getPreferredName()
-                    ),
-                    e
-                );
+                    );
+                    if (e instanceof org.elasticsearch.cluster.block.ClusterBlockException) {
+                        return baseMessage + ", the results index should be writable to detele the model snapshot";
+                    }
+                    return baseMessage;
+                }, e);
             }), () -> runAfter.accept(null)));
     }