Make additional check for connectivity before starting services

The error fragment that appears when starting the mailbox and the device is offline is only shown once the Tor plugin notices it is offline. After a discussion in the chat we came to the conclusion that it might suffice to leave it like that as the fragment appears rather quickly. During some testing I found that it can take 30 seconds for the Tor plugin to start (as in the service is up, not as in bootstrapped and hidden service published etc). Hence it can also take longer than I argued in that discussion for the error fragment to show up. So I think we might want to implement another check before starting services after all.