Error Message Might Not Be Understandable To User

[KhoonSun47]

Issue is on the error message when trying to mark a student absent with a negative index. The user might not understand what a "non-zero unsigned integer" means.

For example:

• Current Input:
  ma w/13 abs/-1

• Current Output:
  Index is not a non-zero unsigned integer.

• Expected Output:
  Index should be invalid because application only accept positive index.