Add processing of soft errors

When processing soft error we only save error to particular subscriber and continue sending.

[MAILPOET-1154]

tests/unit/Cron/Workers/SendingQueue/SendingErrorHandlerTest.php

@@ -0,0 +1,56 @@
+<?php
+namespace MailPoet\Test\Cron\Workers;
+
+use Codeception\Stub;
+use Codeception\Stub\Expected;
+use MailPoet\Cron\Workers\SendingQueue\SendingErrorHandler;
+use MailPoet\Mailer\MailerError;
+use MailPoet\Mailer\SubscriberError;
+use MailPoet\Tasks\Sending as SendingTask;
+
+class SendingErrorHandlerTest extends \MailPoetTest {
+
+  /** @var SendingErrorHandler */
+  private $error_handler;
+
+  function _before() {
+    $this->error_handler = new SendingErrorHandler();
+  }
+
+  function testItShouldProcessSoftErrorCorrectly() {
+    $subscribers = [
+      'john@doe.com',
+      'john@rambo.com',
+    ];
+    $subscriber_ids = [1, 2];
+    $subscriber_errors = [
+      new SubscriberError('john@doe.com', 'Subscriber Message'),
+      new SubscriberError('john@rambo.com', null),
+    ];
+    $error = new MailerError(
+      MailerError::OPERATION_SEND,
+      MailerError::LEVEL_SOFT,
+      'Error Message',
+      null, $subscriber_errors
+    );
+
+    $sending_task = Stub::make(
+      SendingTask::class,
+      [
+        'saveSubscriberError' => Expected::exactly(
+          2,
+          function($id, $message) {
+            if($id === 2) {
+              expect($message)->equals('Error Message');
+            } else {
+              expect($message)->equals('Subscriber Message');
+            }
+          }
+        ),
+      ],
+      $this
+    );
+
+    $this->error_handler->processError($error, $sending_task, $subscriber_ids, $subscribers);
+  }
+}