Validate that configurable system page path aliases are available in all languages

Problem/Motivation

As of #1503146: Aliased paths cannot be set as front page, it is possible to set an unaltered path alias for the front page, 404, or 403 pages. That path will be used as-is, not converted down to a system path.

But that introduces brittleness with multilingual: if your front page is /start in English and /allons-y in French, and you have /start set as the front page path, what will happen when you visit the front page?

To my knowledge, you'll go to /start. The system.site:page paths are not translatable config items anyway, so there'd be no way to have them differ by language. But if the /start alias doesn't exist in French...weirdness could happen.

We should validate that any path you set for the front page or the 4xx pages, if it is an alias, is available in all current languages.