Add error screen for when device is offline

Closes #95 (closed)

Test instructions @earthlingIB:

• reset Mailbox app data
• disable Wifi
• start Mailbox
• skip onboarding
• if prompted, grant app additional permissions
• expected: progress spinner comes up: "starting services", "starting tor"
• expected: error message appears: "device offline?"
• enable Wifi
• expected: after a few seconds max, the error message disappears automatically and the progress spinner re-appears
• expected Mailbox starts up as expected and shows QR code eventually

Test variation:

• do the same as above
• after the progress spinner re-appeared, disable Wifi again, making the error screen appear once more
• enable Wifi once more. As before, the progress spinner should re-appear automatically and the QR code should show up eventually.