Update DynamoDBEnhanced publisher docs to make it clear how to handle errors. (#3449)

Author: millems

services-custom/dynamodb-enhanced/src/main/java/software/amazon/awssdk/enhanced/dynamodb/model/BatchGetResultPagePublisher.java

@@ -36,6 +36,10 @@
  * batchGetResultPagePublisher.subscribe(page -> {
  *     page.resultsForTable(firstItemTable).forEach(item -> System.out.println(item));
  *     page.resultsForTable(secondItemTable).forEach(item -> System.out.println(item));
+ * }).exceptionally(failure -> {
+ *     System.err.println("Failure occurred in subscription.");
+ *     failure.printStackTrace();
+ *     return null;
  * });
  * }
  * </pre>
@@ -44,8 +48,25 @@
  * 2) Subscribing to results across all pages.
  * <pre>
  * {@code
- * batchGetResultPagePublisher.resultsForTable(firstItemTable).subscribe(item -> System.out.println(item));
- * batchGetResultPagePublisher.resultsForTable(secondItemTable).subscribe(item -> System.out.println(item));
+ * CompletableFuture<Void> resultFuture1 =
+ *     batchGetResultPagePublisher.resultsForTable(firstItemTable)
+ *                                .subscribe(item -> System.out.println(item));
+ *
+ * CompletableFuture<Void> resultFuture2 =
+ *     batchGetResultPagePublisher.resultsForTable(secondItemTable)
+ *                                .subscribe(item -> System.out.println(item));
+ *
+ * resultFuture1.exceptionally(failure -> {
+ *     System.err.println("Failure occurred in results for table " + firstItemTable);
+ *     failure.printStackTrace();
+ *     return null;
+ * });
+ *
+ * resultFuture2.exceptionally(failure -> {
+ *     System.err.println("Failure occurred in results for table " + secondItemTable);
+ *     failure.printStackTrace();
+ *     return null;
+ * });
  * }
  * </pre>
  */

services-custom/dynamodb-enhanced/src/main/java/software/amazon/awssdk/enhanced/dynamodb/model/PagePublisher.java

@@ -31,7 +31,11 @@
  * {@code
  *
  * PagePublisher<MyItem> publisher = mappedTable.scan();
- * publisher.subscribe(page -> page.items().forEach(item -> System.out.println(item)));
+ * publisher.subscribe(page -> page.items().forEach(item -> System.out.println(item)))
+ *          .exceptionally(failure -> {
+ *              failure.printStackTrace();
+ *              return null;
+ *          });
  * }
  * </pre>
  *
@@ -41,7 +45,12 @@
  * {@code
  *
  * PagePublisher<MyItem> publisher = mappedTable.scan();
- * publisher.items().subscribe(item -> System.out.println(item));
+ * publisher.items()
+ *          .subscribe(item -> System.out.println(item))
+ *          .exceptionally(failure -> {
+ *              failure.printStackTrace();
+ *              return null;
+ *          });
  * }
  * </pre>
  *