Guard against no start template found in make_stats_object_for_page

The method make_stats_object_for_page iterates over the templates
present on the page, and builds a PropertyStatistics object.

T240312 describes a situation where the start template could not be found.

This is an almost impossible situation, because starting Integraality,
whether via link or via weekly batch update, pre-supposes the presence
of that template. This can only be assumed to be a pywikibot issue,
where a special syntax confuses templatesWithParams.

For now, let's raise an Exception and fail gracefully when this happens.

Bug: T240312