(WIP) Don't throw fatal errors when cert_override.txt sync fails

[JeremyRand]

Instead just stop resolving domains until the error goes away. This fixes issues where ncdns would exit if, for example, Namecoin Core wasn't running or wasn't fully synced when ncdns was launched.

[JeremyRand]

I haven't yet tested whether this breaks anything else, hence the (WIP) label.