* SyncFatalError: show nag instead of hard-crashing.
## Issue
When sync fails, we crash the app.
## Ticket
Maybe closes 39 "Better handle both internal and web backend interruptions / downtime"
## Approach
I'm tackling this from the standpoint that (1) sync errors are not that fatal -- we'll just lost a few recent changes (2) network disconnection is the common cause.
## Changes
- If we are offline:
- Inform user through a nag. All other status is meaningless if we are offline.
- If we are online:
- If api is STATUS_DOWN, show the existing crash page.
- If there is a sync error, show a nag saying settings are now potentially unsynchronized, and add a button to retry sync.
- If there is a chunk error, nag to reload.
* Attempt to detect `status=DOWN`
Previous code resolves the status to either "ok" or "not", which makes the app unable to differentiate between the "degraded" (nag) and "down" (crash) states.
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