Do not process _quant403 and _quant404 if they don't exist

Found some other things while fixing this:

🐛 Translated home pages with path prefixes are not handled Active
✨ Support translated 403 and 404 pages Postponed

Work is happening here:

https://github.com/quantcdn/drupal/pull/223

Kristen Pol → credited steveworley → .

This has been merged in GitHub and will be synced to the d.o repo as part of the next release.

Automatically closed - issue fixed for 2 weeks with no activity.

Unassigning.

Part of 1.6.1 release → .