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.

(cherry picked from commit 6dc8b0b2c6fb2e628356f8dc5c5de4dfd919ec5d)