Inaccurate exception handling message for settle command

[YoengKokLeong]

when settle is called on someone who is not a debtor but loaned money, the exception message of invalid transaction format does not really explain the error in the logic

[nus-se-script]

Team's Response

We agree that the exception message could have been made clearer for this case.

However, the user guide did mention: "The MEMBER should be a valid debtor in the group (i.e. the member should owe money to other members)."

Items for the Tester to Verify

:question: Issue severity

Team chose [severity.VeryLow] Originally [severity.Low]

• [ ] I disagree

Reason for disagreement: [replace this with your explanation]

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