Do not revert if repayAmount > debtAmount

[PaulRBerg]

The current behaviour of the protocol is that it reverts if the repayAmount is bigger than the debtAmount, when calling repayBorrow:

https://github.com/hifi-finance/hifi-protocol/blob/main/contracts/core/balanceSheet/BalanceSheetV1.sol#L565-L569

But this is bad UX. A user may accidentally attempt to withdraw more debt that they have. They should simply be refunded the surplus instead of reverting the execution of the contract call.