Notifications fail silently

[fritzmg]

Currently if the MailerGateway for example returns a Receipt for an unsuccessful delivery, these failed attempts are not logged anywhere at all.

Should we at least add a system log entry in NotificationCenter::sendParcel() for unsuccessful receipts? This would restore the behavior of NC 1.x (and in Contao 5 these logs would be written to the file log as well).

wdyt?

[Toflar]

Yeah I think I would accept a PR that implements a simple listener for that. The ReceiptEvent can be used for exactly that and log to the system log in case the delivery failed. All the info required for that are part of the Receipt :)

[fritzmg]

Sounds good 👍. Will do a PR when I have the time.

[fritzmg]

See #337