18 lines (15 with data), 529 Bytes

{% if messages[0]['role'] == 'system' %}
    {% set offset = 1 %}
{% else %}
    {% set offset = 0 %}
{% endif %}

{{ bos_token }}
{% for message in messages %}
    {% if (message['role'] == 'user') != (loop.index0 % 2 == offset) %}
        {{ raise_exception('Conversation roles must alternate user/assistant/user/assistant/...') }}
    {% endif %}

    {{ '### ' + message['role'] | capitalize + ':\n' + message['content'] | trim + '\n\n' }}
{% endfor %}

{% if add_generation_prompt %}
    {{ '### Assistant:\n' }}
{% endif %}