Log uncaught exceptions from scheduled once tasks

`ScheduledThreadPoolExecutor` allows you to schedule tasks to run once or periodically at the future. If such a task throws an exception, that exception is caught and reported in the future that `ScheduledThreadPoolExecutor#schedule` returns. However, we typically do not capture the future / do not test it for errors. This results in exception being swallowed and not reported. To mitigate this we now wrap any command in a LoggingRunnable  (already used for periodic tasks).  Also, RunnableCommand is changed not to swallow exception but percolate them further for reporting by the future.

Closes #15824

core/src/main/java/org/elasticsearch/threadpool/ThreadPool.java

@@ -357,7 +357,7 @@ public class ThreadPool extends AbstractComponent {
         if (!Names.SAME.equals(name)) {
             command = new ThreadedRunnable(command, executor(name));
         }
-        return scheduler.schedule(command, delay.millis(), TimeUnit.MILLISECONDS);
+        return scheduler.schedule(new LoggingRunnable(command), delay.millis(), TimeUnit.MILLISECONDS);
     }
 
     public void shutdown() {
@@ -633,6 +633,7 @@ public class ThreadPool extends AbstractComponent {
                 runnable.run();
             } catch (Throwable t) {
                 logger.warn("failed to run {}", t, runnable.toString());
+                throw t;
             }
         }