frontends: use new EXIT_BACKEND_NOT_FOUND exit code: - new EXIT_BACKEND_NOT_FOUND (5) is used when frontend can't find backend with the bridge. It differs from EXIT_BRIDGE_ERROR (3) which is used when something went wrong when connecting to backend. - use this exit code in quick app and in jp

#!/bin/bash
set -e

usermod -u "$(stat -c %u /var/lib/prosody/.)" prosody

if [[ "$1" != "prosody" ]]; then
    exec prosodyctl "$@"
    exit 0;
fi

if [ "$LOCAL" -a  "$PASSWORD" -a "$DOMAIN" ] ; then
    prosodyctl register "$LOCAL" "$DOMAIN" "$PASSWORD"
fi

exec runuser -u prosody -- "$@"