Add dedicated security error for OngoingFork

[gregdhill]

Transaction verification is disabled if we have an OngoingFork with more confirmations + the security parameter than the main chain (see here). This is not reflected in the security module and thus our client-side code is unable to determine if it is valid to execute a payment. We should reflect this possible downtime in the security pallet to also avoid unintended lapses in the expiry periods.

[gregdhill]

We no longer use the global security errors, see the (now closed) linked PR.