Feature request: app-level errors shouldn't affect container health

[SlavikCA]

The software should differentiate between container errors and app errors.

For example, if user try to access http://ddns-updater/invalid-url - that's application-level error, it should be logged, but should not result in the container health degradation.

I think the container healthcheck should be downgraded in cases, such as lack of network connectivity (can't response to DNS requests), storage issues (can't read /write) config files, etc.

But app-level errors (such as error response from Cloudflare API) should be logged, displayed in UI, but not affect container health.

Currently (v2.6.0), when ddns-updater receives error response for API request, the container state changes to unhealthy, which makes UI unaccessible on system, where routing is done via Traefik, because Traefik doesn't route to unhealthy containers.