Fix Mailer error messages not displaying in previews [MAILPOET-1662]

lib/API/JSON/v1/Newsletters.php

@@ -358,7 +358,7 @@ class Newsletters extends APIEndpoint {
         if($result['response'] === false) {
           $error = sprintf(
             __('The email could not be sent: %s', 'mailpoet'),
-            $result['error_message']
+            $result['error']->getMessage()
           );
           return $this->errorResponse(array(APIError::BAD_REQUEST => $error));
         } else {

tests/integration/API/JSON/v1/NewslettersTest.php

@@ -799,7 +799,12 @@ class NewslettersTest extends \MailPoetTest {
             expect($subscriber)->equals($subscriber);
             return array(
               'response' => false,
-              'error_message' => 'failed'
+              'error' => Stub::make(
+                '\MailPoet\Mailer\MailerError',
+                array(
+                  'getMessage' => 'failed'
+                )
+              )
             );
           }
         )