rec: A ServFail while retrieving DS/DNSKEY records is just that

rec: A ServFail while retrieving DS/DNSKEY records is just that

Before that commit, failing to get the DS or DNSKEY records needed
during validation because of a network issue would trigger a Bogus
DNSSEC validation result because validation could not be performed,
but that should just be a Server Failure instead.
This is especially an issue because the Bogus result would get
inserted into the cache and could stay there for as long as
'max-cache-bogus-ttl' seconds.