Bug 951775 - Add a list of pages with errors to the wiki

This is an attempt to make the black box of KumaScript errors visible to us.
What this PR does:
* A new view is introduced here: https://developer-local.allizom.org/en-US/docs/errors
* A document counter has been added to the document lists:
  * https://developer-local.allizom.org/en-US/docs/all
  * https://developer-local.allizom.org/en-US/docs/templates
  * https://developer-local.allizom.org/en-US/docs/tag/test
* The exclude rules have been updated to match with the search https://github.com/mozilla/kuma/pull/1923.