Prevent user to accept the manual signature of proc contract unless the signed document is attached

Summary:

We are using manual signature for procurement contracts. After contract is fully approved, status changes from "Pending Approval" to "Pending Signature".

Once the user accepts the signature by selecting (Sign Contract) from the action list and click "Ok", status changes from "Pending Signature" to "Active".

Sometimes, after contract is physically signed off, buyers accept the signature in the system without attaching the signed document under Documents Tab "Signed Contract Document".

We need to prevent users from accepting the signature unless they have attached the signed document.