Associate validation message correctly with set of checkboxes (fieldset)

[simonrjones]

When a set of checkboxes have an error screen readers don't read out the error message.

On the test form we associated the error message with the fieldset.

On the live form we do not do this, this needs updating.

Live form:

<fieldset class="tbxforms-fieldset">
                <legend class="tbxforms-fieldset__legend">
                    <h2 class="heading--3">Area</h2>
                </legend>

                                    <span id="id_area_error" class="tbxforms-error-message">Please select at least one area.</span>

Static test form:

<fieldset id="category" class="tbxforms-fieldset" aria-describedby="id_category_hint id_category_invalid_error">
                    <legend class="tbxforms-fieldset__legend">
                        <h2 class="rnib-form-label rnib-form-label__wrapper">Category (required)</h2>
                    </legend>

                                            <span id="id_category_invalid_error" class="tbxforms-error-message">Error: The category field is required.</span>
                    
                    <div id="id_category_hint" class="rnib-form-hint">Select at least one option</div>

[gtrinkwon]

I've adjusted the Twig and think this should be correct in the next release branch, will need verifying with RNIB