Logging: Add logging for failed TTL purges

In order to get some information if the TTL purger thread could
successfully delete all documents per bulk exection, this commit
adds some logging. TRACE level logging will potentially contain
a lot of information about all the bulk failures.

Closes elastic#11019

src/main/java/org/elasticsearch/indices/ttl/IndicesTTLService.java

@@ -26,6 +26,7 @@
 import org.apache.lucene.search.SimpleCollector;
 import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.action.ActionListener;
+import org.elasticsearch.action.bulk.BulkItemResponse;
 import org.elasticsearch.action.bulk.BulkRequest;
 import org.elasticsearch.action.bulk.BulkResponse;
 import org.elasticsearch.action.bulk.TransportBulkAction;
@@ -280,12 +281,28 @@ private BulkRequest processBulkIfNeeded(BulkRequest bulkRequest, boolean force)
                 bulkAction.executeBulk(bulkRequest, new ActionListener<BulkResponse>() {
                     @Override
                     public void onResponse(BulkResponse bulkResponse) {
-                        logger.trace("bulk took " + bulkResponse.getTookInMillis() + "ms");
+                        if (bulkResponse.hasFailures()) {
+                            int failedItems = 0;
+                            for (BulkItemResponse response : bulkResponse) {
+                                if (response.isFailed()) failedItems++;
+                            }
+                            if (logger.isTraceEnabled()) {
+                                logger.trace("Bulk deletion failures for [{}]/[{}] items, failure message: [{}]", failedItems, bulkResponse.getItems().length, bulkResponse.buildFailureMessage());
+                            } else {
+                                logger.error("Bulk deletion failures for [{}]/[{}] items", failedItems, bulkResponse.getItems().length);
+                            }
+                        } else {
+                            logger.trace("Bulk deletion took " + bulkResponse.getTookInMillis() + "ms");
+                        }
                     }
 
                     @Override
                     public void onFailure(Throwable e) {
-                        logger.warn("failed to execute bulk");
+                        if (logger.isTraceEnabled()) {
+                            logger.trace("failed to execute bulk", e);
+                        } else {
+                            logger.warn("failed to execute bulk: [{}]", e.getMessage());
+                        }
                     }
                 });
             } catch (Exception e) {