mod_report_forward: Fix traceback on error message passed to datetime function If archive:get() returns an error, e.g. because it can't find a message with the given ID, it may return e.g. nil, "item-not-found" or similar where the error condition ends up in reported_message_time, which gets passed to util.datetime, which thus ends up doing math to a string or error object.

[Install]
WantedBy=sockets.target

[Socket]
ListenStream=0.0.0.0:5222
ListenStream=5222
BindIPv6Only=ipv6-only
Accept=false
Service=prosody.service