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/Mailer/MailerError.php

@@ -73,6 +73,13 @@ class MailerError {
     return $this->subscribers_errors;
   }
 
+  /**
+   * Temporary method until we implement UI for subscriber errors
+   */
+  function switchLevelToHard() {
+    $this->level = self::LEVEL_HARD;
+  }
+
   function getMessageWithFailedSubscribers() {
     $message = $this->message ?: '';
     if(!$this->subscribers_errors) {