Issue #3379107: Simple cron should log failed manual executions.

src/Commands/SimpleCronCommands.php

@@ -99,6 +99,9 @@ class SimpleCronCommands extends DrushCommands {
     if ($status) {
       $this->getLogger('simple_cron')->notice('Execution of cron job @job is completed.', ['@job' => $job->id()]);
     }
+    else {
+      $this->getLogger('simple_cron')->error('Execution of cron job @job failed.', ['@job' => $job->id()]);
+    }
   }
 
   /**