It was used only to prevent appending unprocessed subscribers into error message.
Since the message is now composed by on demand by MailerError the parameter is not needed any more.
[MAILPOET-1154]
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]
