Prevent transferOwnership from failing

[makoto]

Currently there is a chance that transferOwnership to be handed over to wrong address or empty address. If that happens the entire deposit will be stack forever.

The possible solution could be

• Allow having more than one owner.
• Force the new user to claim (like this). This is more secure but it will require two transactions (one to transfer, another to claim) so it would become a bit inconvenient.