Set payload response in happy path of ReplayCommitLogsToSqlAction (#1229)

* Set payload response in happy path of ReplayCommitLogsToSqlAction

I suspect this may be the reason the logs are missing on the happy path (when it
runs successfully), but are visible on the exception paths (which do set the
payload response). I don't think App Engine likes it when a Web request
terminates without a response.

This also adds more logging and error handling.

core/src/test/java/google/registry/backup/ReplayCommitLogsToSqlActionTest.java

@@ -494,6 +494,8 @@ public class ReplayCommitLogsToSqlActionTest {
   private void runAndAssertSuccess(DateTime expectedCheckpointTime) {
     action.run();
     assertThat(response.getStatus()).isEqualTo(SC_OK);
+    assertThat(response.getPayload())
+        .isEqualTo("ReplayCommitLogsToSqlAction completed successfully.");
     assertThat(jpaTm().transact(SqlReplayCheckpoint::get)).isEqualTo(expectedCheckpointTime);
   }