DNS node better handling of corrupt master file

[dangershony]

From discord by sondreb:

If the masterfile.json is invalid JSON (written during a crash or something), then at startup this is logged:

[2019-10-22 19:07:24.1226 15] INFO: Stratis.Bitcoin.Features.Dns.DnsSeedServer.Initialize Loading cached DNS masterfile from

And then:

[2019-10-22 19:07:24.1368 7] INFO: Stratis.Bitcoin.Features.Dns.DnsSeedServer.SeedMasterFile Seeding DNS masterfile with SOA and NS resource records

Which it logs again and again and again, without exception logged. So this scenario must be fixed and properly logged as exception.