Detect broken Internet connection

When a WAN is down and Internet connection is not available, updater "spam" Foris notifications with:

unreachable: https://repo.turris.cz/omnia/lists/base.lua: Couldn't resolve host 'repo.turris.cz'

which is a bit cryptic, especially for new Turris users.

It would be really nice to enhance user experience and lower "false issues" like that (that's really not unique).