set error on field emailCode instead of global

[eitchugo]

This sets a field in the challenge method, enabling the template to check for errors in input emailCode. Without this change, the error message was not appearing in the template, called by this code:

                <#if messagesPerField.existsError('emailCode')>
                    <span id="input-error-otp-code" class="${properties.kcInputErrorMessageClass!}"
                          aria-live="polite">
                        ${kcSanitize(messagesPerField.get('emailCode'))?no_esc}
                    </span>
                </#if>