wrong error message on add appointment

[nus-pe-bot]

In line with "Cannot add appointment" bug report, no appointment was made between John Doe(patient) and George(Doctor) on that day, but George do have an appointment already, perhaps add different exception for this kind of input.

---

_{[original: nus-cs2113-AY2122S2/pe-interim#584] [original labels: type.FunctionalityBug severity.VeryLow]}

[shxr3f]

Team's Response

Yes we understand that a different error message could be helpful to the user in identifying what went wrong!

Duplicate status (if any):

--