Improve DB integrity on financial_account.parent_id

Create a contact. Create a financial account with that contact as the parent_id. Soft delete the contact - note that there are no warning or error messages.

Solution
An attempt to soft-delete a contact referenced from a record in financial_account table should fail with message like 'One or more Financial Accounts is owned by the contact you are trying to delete. Please remove such references before deleting this contact.' If an attempt is about to be made to delete this kind of contact as part of merge/delete operation, it should also fail with the same message.