Payable modifier should throw

[czepluch]

IMO a modifier checking a value on a payable function should throw and not return. Currently people paying 0.029 ether would not get notified that their attempt was unsuccessful and 0.029 ether would just be transferred to the smart contract. Transferring another 0.001 ether would not make the request successful. So I suggest that this (https://github.com/paritytech/contracts/blob/58842b9/SMSVerification.sol#L57) line should throw instead of returning.