Bounce undeliverable message and presence stanzas

When a message or presence stanza cannot be delivered, an error should be bounced back to the original sender.

Currently, this scenario gets handled by the offline message storage, which is not always appropriate. When, for instance, an anonymous user sends a message to an entity on a remote domain, delivery is never going to work (and storing an outbound message in offline storage would not have made sense in the first place).