Let systemd restart a crashed XWiki and send the crash notification

check-proc polls a pid file every minute to find out that the JVM died. The Debian packages run XWiki under systemd (tomcatN.service, xwiki.service), which has no such pid file, knows immediately when the process dies, and already restarts it (Restart=on-abort).

Rely on systemd for restarting a dead process and use OnFailure= to send the notification with diagnostics. Keep polling only for what systemd cannot see, such as a hung JVM.