Don't show notification on failed sync

[schiessle]

For me it happens regularly that a sync fails because of bad network connection. In this case I always get a notification on my phone. This information is useless and disturbing, I can' t do anything, but only hope that the next time the app tries to sync the connection is better and it will work. Therefore I think it would be good if the app wouldn't show any notification but just try again.

The only situation it might be useful is for debugging purpose. So maybe a config switch to enable notification on sync errors might make sense, disabled by default.