Refactor sending methods to use error mappers

We want to add some logic to error handling.
This commit extracts error handling code from sending methods classes,
which already do a lot of other stuff, to error mappers which are responsible
for creating proper error object (MailerError). This error object is a replacement
for assoc. array which already had some special keys for certain usecases and
can not be properly type hinted.

[MAILPOET-1154]

lib/Mailer/Methods/ErrorMappers/ConnectionErrorMapperTrait.php

@@ -0,0 +1,14 @@
+<?php
+namespace MailPoet\Mailer\Methods\ErrorMappers;
+
+use MailPoet\Mailer\MailerError;
+
+trait ConnectionErrorMapperTrait {
+  function getConnectionError($message) {
+    return new MailerError(
+      MailerError::OPERATION_CONNECT,
+      MailerError::LEVEL_HARD,
+      $message
+    );
+  }
+}