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/SendGridMapper.php

@@ -0,0 +1,19 @@
+<?php
+namespace MailPoet\Mailer\Methods\ErrorMappers;
+
+use MailPoet\Mailer\MailerError;
+use MailPoet\Mailer\Mailer;
+
+class SendGridMapper {
+  use ConnectionErrorMapperTrait;
+
+  function getErrorFromResponse($response, $subscriber, $extra_params) {
+    $response = (!empty($response['errors'][0])) ?
+      $response['errors'][0] :
+      sprintf(__('%s has returned an unknown error.', 'mailpoet'), Mailer::METHOD_SENDGRID);
+    if(empty($extra_params['test_email'])) {
+      $response .= sprintf(' %s: %s', __('Unprocessed subscriber', 'mailpoet'), $subscriber);
+    }
+    return new MailerError(MailerError::OPERATION_SEND, MailerError::LEVEL_HARD, $response);
+  }
+}