Failure to retrieve DNSKEYs of an Insecure zone should not be fatal.

Failure to retrieve DNSKEYs of an Insecure zone should not be fatal.

This issue happens if a record set is signed even though the zone
itself is Insecure. Syncres then tries to retrieve DNSKEYs and a
timeout on that would lead to an ImmediateServFailException.

Only throw exception later in validateRecordsWithSigs, after checking
zone cuts, when we are sure the zone is Secure.