Temporarily set every sending error to hard

This is temporary solution until we implement follow-up feature https://mailpoet.atlassian.net/browse/MAILPOET-1155

[MAILPOET-1154]

lib/Cron/Workers/SendingQueue/SendingQueue.php

@@ -206,6 +206,8 @@ class SendingQueue {
     if($send_result['response'] === false) {
       $error = $send_result['error'];
       assert($error instanceof MailerError);
+      // Always switch error level to hard until we implement UI for individual subscriber errors
+      $error->switchLevelToHard();
       $this->error_handler->processError($error, $sending_task, $prepared_subscribers_ids, $prepared_subscribers);
     }
     // update processed/to process list