Reject row number 0 as invalid input

[EricccTaiwan]

This update adds validation to reject row number 0, which was previously allowed. While row numbers exceeding BOARD_SIZE were already handled, there was no explicit check for 0. An error is now triggered when 0 is entered.

Before

$ ./ttt
 1 |             
 2 |             
 3 |             
 4 |             
---+-------------
      A  B  C  D
X> a0
X> b0
X> a5
Invalid operation: index exceeds board size
X> ^C
$

After

$ ./ttt
 1 |             
 2 |             
 3 |             
 4 |             
---+-------------
      A  B  C  D
X> a0
Invalid operation: index cannot be 0
X> b0
Invalid operation: index cannot be 0
X> a5
Invalid operation: index exceeds board size
X> ^C
$ 

[EricccTaiwan]

This update adds validation to reject row number 0, which was previously allowed. While row numbers exceeding BOARD_SIZE were already handled, there was no explicit check for 0. An error is now triggered when 0 is entered.

I also wanted to ask whether it would be better to use a single error message for both row number 0 and values exceeding BOARD_SIZE, rather than handling them separately.

[jserv]

Thank @EricccTaiwan for contributing!