Notify when celery-weblate service is down

Weblate can take a lot of resources on the machine in case of language deletion for example (happened lately with zh_HANS language issue), which could cause an OOM on celery-weblate systemd service.
This service is however necessary to perform automatically some operations on Weblate: when it crashes it means we don't receive anymore the pull requests for translations.

So it would be really nice to get an email whenever it happens.