Is your feature request related to a problem? Please describe.
If scheduled backups fail silently in the background, users might not notice (for instance, they're not looking into the log files) and are without backups for days or weeks.

Describe the solution you'd like
Every OS these days has support for notifications and it would be great if backrest would add support for that. The feature list mentions notification support for third-party services like Discord or Slack, but a local notification would be useful as well, esp. as default value for a new configuration.

Notifications could be sent when a backup finishes, fails or misses a variable amount of runs.

Additional context
I'm currently using custom scripts to create those notifications on Linux and Windows but that is not a viable solution to a lot of users.