rec: Backport 11890 to rec-4.7.x: Failure to retrieve DNSKEYs of an Insecure zone should not be fatal.

[omoerbeek]

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 6dc8b0b)

Backport of #11890

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