More robust when a nameserver does not reply. Closes #1

Written by Alexandre Pion and tested by me.

% ./ns-tree.py eu.org           
Warning: could not resolve ns3.keltia.net. for zone eu.org.
eu.org
eu.org.
  ├─ eu.org. ← { ns.bortzmeyer.eu.org., ns1.eu.org. }
  ├─ wolfhugel.eu. ← { gra.wolfhugel.eu. }
  │  ├─ wolfhugel.eu. ← { roubaix.wolfhugel.eu., armoire.wolfhugel.eu., gra.wolfhugel.eu. }
  │  └─ eu.org. ← { ns1.eu.org. }
  ├─ eriomem.net. ← { ns1.eriomem.net. }
  │  ├─ eu.org. ← { ns1.eu.org. }
  │  └─ eriomem.net. ← { ns1.eriomem.net. }
  ├─ bsd-dk.dk. ← { hobbes.bsd-dk.dk. }
  │  └─ one.com. ← { ns02.one.com., ns01.one.com. }
  │     ├─ g1-dns.one. ← { auth.g1-dns.one. }
  │     │  ├─ g1-dns.one. ← { auth.g1-dns.one. }
  │     │  └─ g1-dns.com. ← { auth.g1-dns.com. }
  │     │     ├─ g1-dns.com. ← { auth.g1-dns.com. }
  │     │     └─ g1-dns.one. ← { auth.g1-dns.one. }
  │     └─ g1-dns.com. ← { auth.g1-dns.com. }
  ├─ billaud.eu.org. ← { wombat.billaud.eu.org. }
  │  └─ billaud.eu.org. ← { microtus.billaud.eu.org., wombat.billaud.eu.org., ns2.billaud.eu.org., wapiti.billaud.eu.org. }
  └─ elm.net. ← { dns.elm.net. }
     ├─ badassops.com. ← { stitch.dns.badassops.com. }
     │  ├─ badassops.com. ← { lilo.dns.badassops.com., stitch.dns.badassops.com. }
     │  └─ elm.net. ← { dns.elm.net. }
     ├─ elm.net. ← { dns.elm.net. }
     └─ at-the-beach.nl. ← { sin.at-the-beach.nl., sun.at-the-beach.nl. }
        ├─ elm.net. ← { dns.elm.net. }
        ├─ badassops.com. ← { stitch.dns.badassops.com. }
        └─ at-the-beach.nl. ← { sin.at-the-beach.nl., sun.at-the-beach.nl. }