Replace "xwiki.virtual.redirect" with an error template(or page) just like the other errors

The current "xwiki.virtual.redirect" configuration is not very practical, specially since it's enabled by default. Basically it applies only for a standard local install, any remote DNS access is impossible.

A better option is to use a vm error template, just like we do for non-existing documents and also provide the ability to override that template from the wiki (through a document like XWiki.WikiDoesNotExist).

However, even if most deployed installs already rely on the autowww feature (use an address like "www.domain.com") and may not have problems accessing their wiki, by default we should serve the main wiki when the requested wiki does not exist. To get an error instead, the admin can set a configuration property "xwiki.virtual.failOnWikiDoesNotExist" to "1".

This issue should help on the transition introduced by XWIKI-8822.