Reindex: do not log when can't clear old scroll (#22942)

Versions of Elasticsearch prior to 2.0 would return a scroll id
even with the last scroll response. They'd then automatically
clear the scroll because it is empty. When terminating reindex
will attempt to clear the last scroll it received, regardless of
the remote version. This quiets the warning when the scroll cannot
be cleared for versions before 2.0.

Closes #22937

modules/reindex/src/main/java/org/elasticsearch/index/reindex/remote/RemoteScrollableHitSource.java

@@ -120,10 +120,23 @@ public class RemoteScrollableHitSource extends ScrollableHitSource {
             }
 
             @Override
-            public void onFailure(Exception t) {
+            public void onFailure(Exception e) {
-                logger.warn((Supplier<?>) () -> new ParameterizedMessage("Failed to clear scroll [{}]", scrollId), t);
+                logFailure(e);
                 onCompletion.run();
             }
+
+            private void logFailure(Exception e) {
+                if (e instanceof ResponseException) {
+                    ResponseException re = (ResponseException) e;
+                    if (remoteVersion.before(Version.V_2_0_0) && re.getResponse().getStatusLine().getStatusCode() == 404) {
+                        logger.debug((Supplier<?>) () -> new ParameterizedMessage(
+                                "Failed to clear scroll [{}] from pre-2.0 Elasticsearch. This is normal if the request terminated "
+                                        + "normally as the scroll has already been cleared automatically.", scrollId), e);
+                        return;
+                    }
+                }
+                logger.warn((Supplier<?>) () -> new ParameterizedMessage("Failed to clear scroll [{}]", scrollId), e);
+            }
         });
     }
 
@@ -134,7 +147,7 @@ public class RemoteScrollableHitSource extends ScrollableHitSource {
         threadPool.generic().submit(() -> {
             try {
                 client.close();
-                logger.info("Shut down remote connection");
+                logger.debug("Shut down remote connection");
             } catch (IOException e) {
                 logger.error("Failed to shutdown the remote connection", e);
             }