[BUGFIX] Handle errors while sending mails correctly without exceptions, Visualize errors inside the backend

[FEATURE] Allow for adding files from path

See merge request !41

# Conflicts:
#   Classes/Service/MailTemplateService.php
#   composer.json
#   ext_emconf.php

This feature is only usable if the e-mail will be sind right away,
so set $this->setIgnoreMailQueue(TRUE); otherwise, you need to hand
over a correct File and FileReference

https://trello.com/c/kP3J2cmj/3573-sgmail-hook-typo3-exception-typeerror

This commit fixes the following Exception: Argument 3 passed to SGalinski\SgMail\Hooks\ProcessDatamap::processDatamap_afterDatabaseOperations() must be of the type int, string given, called in /var/www/softingit.dev/web/typo3/sysext/core/Classes/DataHandling/DataHandler.php on line 855
The exception was thrown when creating a new translation of a page.