Throw exceptions in unreachable branches instead of having no-op

[anton-trunov]

There is a couple of unreachable branches in the code with comment like won't reach and no-ops. It's safer against future changes to insert throw instructions instead of no-op as in this case if the invariant is violated the state changes will be reverted.

See CalculateTotalWithdrawal and WithdrawalStakeRewards procedures.

[renlulu]

OK, will do that!