App throws an "Sync failed error" even though it is still syncing

[gndelia]

For large sync processes, the app throws a "Sync failed" error, but the "Syncing..." is still visible. If you wait enough, the sync process finishes successfully. The error seems to be thrown when the App loses the connection with the Indexer (I think).

We should show a better error message that doesn't confuse the user. This is not related to #480. This might also be happening in Mobile (which may indicate it's a bug in Core)