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/API/JSON/v1/Mailer.php

@@ -34,7 +34,7 @@ class Mailer 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 {
@@ -46,4 +46,4 @@ class Mailer extends APIEndpoint {
     MailerLog::resumeSending();
     return $this->successResponse(null);
   }
-}
+}