Warn user if no network permission is given

[herzhenr]

If a user doesn't allow network access in the iOS app, no devices are shown in the Discover page and if the user tries to wake devices, there isn't an indication at all that network permission are missing (The app says the owl packets are send successfully even though no package is sent at all, see #19 ).

A check should be performed if network access is allowed and if not an error should pop up instructing the user how to give the permission. In the Add Devices screen a error text could be shown why no devices are shown and a button to give the right permissions