Warn when the server is unavailable rather than just saying "synchronizing"

[ara4n]

Steps to reproduce

1. Block your server somehow (network failure; server offline)
2. Fg the app
3. Watch a neverending "Synchronizing" chip and timeline spinner
4. Wonder why your app won't sync

Outcome

What did you expect?

"Offline" or "Server unavailable" or "Can't connect" or similar.

What happened instead?

Neverending "Synchronizing" spinner, implying that the server is accessible but is taking a long time to sync.

We should be able to distinguish the two (e.g. Threema distinguishes red & orange connectivity state) to inform the user as to what's going on.

Your phone model

No response

Operating system version

No response

Application version and app store

No response

Homeserver

No response

Will you send logs?

No

Are you willing to provide a PR?

No