Handled exception for dns.resolver.NoNameservers

[mazen160]

Summary

When an NS query does not return nameservers, this exception is raised. However, it was not handled, which led to the crash of the program.

This commit handles this exception.

[mazen160]

An example of the exception (for reference)

dnsrecon -d [redacted]
[*] Performing General Enumeration of Domain: [redacted]
[-] All nameservers failed to answer the DNSSEC query for [redacted]
[-] Error while resolving SOA record.
Traceback (most recent call last):
  File "./dnsrecon.py", line 1691, in <module>
    main()
  File "./dnsrecon.py", line 1649, in main
    std_enum_records = general_enum(res, domain, xfr, goo, bing, spf_enum, do_whois, do_crt, zonewalk)
  File "./dnsrecon.py", line 960, in general_enum
    for ns_rcrd in res.get_ns():
  File "/usr/share/dnsrecon/lib/dnshelper.py", line 174, in get_ns
    answer = self._res.query(self._domain, 'NS')
  File "/usr/lib/python2.7/dist-packages/dns/resolver.py", line 947, in query
    raise NoNameservers(request=request, errors=errors)
dns.resolver.NoNameservers: All nameservers failed to answer the query [redacted]. IN NS: Server [redacted] UDP port 53 answered SERVFAIL

[mazen160]

Without terminating the program, another similar exception will be triggered next when sending an MX query.