Handling NoNameservers exception

Handling NoNameservers exception

In the shutdown system test multiple queries are sent to a resolver
instance, in the meantime we terminate the same resolver process for
which the queries were sent to, either via rndc stop or a SIGTERM
signal, that means the resolver may not be able to answer all those
queries, since it has initiated the shutdown process.

The dnspython library raises a dns.resolver.NoNameservers exception when
a resolver object fails to receive an answer from the specified list
of nameservers (resolver.nameservers list), we need to handle this
exception as this is something that may happen since we asked the
resolver to terminate, as a result it may not answer clients even if
an answer is available, as the operation will be canceled.