Exceptions: update error messages

[Zhiyuan-Amos]

PersonNotFoundException can only be initialized using the empty constructor and does not have an error message. Also, DuplicatePersonException can only be initialized with a default error message.

The lack of error message and undescriptive error messages in exceptions aren't helpful for debugging purposes.

I'm suggesting that we can pass in the Person object as a parameter (and we call Person#toString() as the value for the error message) when constructing these exceptions, so that we can know which Person object caused the exception.