Summary: Improve Handling & Messaging During Checkmk Maintenance

During Checkmk backend updates (e.g. stopping a site), users are frequently confronted with red error messages such as “Unhandled exception”.
This creates the impression that Checkmk is broken, even though the behavior is expected during maintenance activities.

Key issues:

- The wording “Unhandled exception” is misleading and alarming for end users.
- Similar messages appear in multiple places and give poor UX signals during planned maintenance, but also in general.
- Existing downtime/maintenance mechanisms do not seem to cover backend or site-level maintenance properly.

Suggested improvement:

Introduce clearer, maintenance-aware status messages when a site or backend is intentionally stopped.
Replace generic technical error messages with user-friendly explanations (e.g. “System is currently under maintenance”).
Ideally provide a dedicated maintenance mode for Checkmk itself, not only for monitored hosts/services.

This would significantly reduce confusion, false incident reports, and improve overall user experience during upgrades and maintenance windows.