Fix: align error message with validation logic in _add_custom_tokens

[abhinavansh18]

BUG FIX #421

Description

This PR corrects a minor inconsistency in the error message for custom token ID validation within the _add_custom_tokens method.

The validation logic correctly checks if an ID is within the range [0, 98], but the ValueError message incorrectly stated the valid range was [1, 98]. This change updates the error message to match the actual validation logic, preventing potential confusion for developers.

This is a static text change in an error message. I have manually reviewed the code to ensure the new message ...is not in [0, 98]. accurately reflects the validation logic if i < 0 or i > 98:.

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