[State of Tic Tac Toe]: Added missing instruction append for raising errors.

exercises/practice/state-of-tic-tac-toe/.docs/instructions.append.md

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+# Instructions append
+
+## Exception messages
+
+Sometimes it is necessary to [raise an exception][raising-exceptions].
+When you do this, you should always include a **meaningful error message** to indicate what the source of the error is.
+This makes your code more readable and helps significantly with debugging.
+For situations where you know that the error source will be a certain type, you can choose to raise one of the [built in error types][error-base-classes], but should still include a meaningful message.
+
+
+A valid game of `Tic Tac Toe` ends when one player wins.
+If both players are in a winning state, or there is a mismatch between the winner and their number of moves, this exercise expects you to use the [raise statement][raise] and "throw" a `ValueError` in your solution.
+Your code is also expected to throw a `ValueError` if one player is assessed as playing two turns in a row, or playing the first move when they were assigned to play the second move.
+
+
+To raise a `ValueError` with a message, write the message as an argument to the `exception` type:
+
+```python
+# Example when player X goes before player O.
+raise ValueError("Wrong turn order: O started")
+
+# Example when player X goes twice.
+raise ValueError("Wrong turn order: X went twice")
+
+# Example when player X wins AND player O wins.
+raise ValueError("Impossible board: game should have ended after the game was won")
+```
+
+The tests will check for an exact error message match, so please read the expected results carefully.
+
+[raising-exceptions]: https://docs.python.org/3/tutorial/errors.html#raising-exceptions
+[error-base-classes]: https://docs.python.org/3/library/exceptions.html#base-classes
+[raise]: https://docs.python.org/3/reference/simple_stmts.html#the-raise-statement