fraud: handle not verified fraud proofs

[vgonkivs]

There are case when we are not able to ensure the Fraud Proof is valid. Mainly, It could happened due to a network issue when we can't fetch an ExtendedHeader. In a private discussion with @Wondertan we discussed several ways to resolve this:

1. start an infinite routine that will fetch header until it will not be available;
2. add proof with Not Verified state to FraudStore;
3. consider this case during FraudSync implementation(#536);

IMO, the second way will make FraudStore structure more complicated. I think we should store only verified and correct Fraud Proofs.