Merge pull request #729 from mailpoet/mailer_endpoint_update

Fixes mailer's error message not being displayed in Settngs [MAILPOET-715]

lib/API/Endpoints/Mailer.php

@@ -24,7 +24,7 @@ class Mailer extends APIEndpoint {
     if($result['response'] === false) {
       $error = sprintf(
         __('The email could not be sent: %s', 'mailpoet'),
-        $result['error']
+        $result['error_message']
       );
       return $this->errorResponse(array(APIError::BAD_REQUEST => $error));
     } else {