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]

tests/unit/Mailer/Methods/SendGridTest.php

@@ -1,6 +1,7 @@
 <?php
 namespace MailPoet\Test\Mailer\Methods;
 
+use MailPoet\Mailer\Methods\ErrorMappers\SendGridMapper;
 use MailPoet\Mailer\Methods\SendGrid;
 
 class SendGridTest extends \MailPoetTest {
@@ -24,7 +25,8 @@ class SendGridTest extends \MailPoetTest {
     $this->mailer = new SendGrid(
       $this->settings['api_key'],
       $this->sender,
-      $this->reply_to
+      $this->reply_to,
+      new SendGridMapper()
     );
     $this->subscriber = 'Recipient <mailpoet-phoenix-test@mailinator.com>';
     $this->newsletter = array(