The tool could be more robust to servers which are down

If one of the name servers is down, the tool crashes completely. May be add an option to have just a warning?

% ./ns-tree.py cyberstructure.fr
Traceback (most recent call last):
  File "/home/stephane/tmp/ns-tree/./ns-tree.py", line 163, in <module>
    tree.build()
  File "/home/stephane/tmp/ns-tree/./ns-tree.py", line 27, in build
    node.create_edges( self.nodes )
  File "/home/stephane/tmp/ns-tree/./ns-tree.py", line 99, in create_edges
    ns_zone = get_zone_name( ns )
              ^^^^^^^^^^^^^^^^^^^
  File "/home/stephane/tmp/ns-tree/./ns-tree.py", line 129, in get_zone_name
    ans = dns.resolver.resolve( qname, dns.rdatatype.SOA, raise_on_no_answer=False )
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/dns/resolver.py", line 1565, in resolve
    return get_default_resolver().resolve(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/dns/resolver.py", line 1321, in resolve
    timeout = self._compute_timeout(start, lifetime, resolution.errors)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/dns/resolver.py", line 1075, in _compute_timeout
    raise LifetimeTimeout(timeout=duration, errors=errors)
dns.resolver.LifetimeTimeout: The resolution lifetime expired after 5.401 seconds: Server Do53:192.168.2.254@53 answered The DNS operation timed out.; Server Do53:192.168.2.254@53 answered The DNS operation timed out.; Server Do53:192.168.2.254@53 answered The DNS operation timed out.