Generate requirements error when language negotiation set to "language fallback"

This breaks boost because language negotiation is then no longer URL based.

An anonymous user with their browser set to a language other than the site's default language will navigate to a non-cached page. The page will be displayed in the user's browser language. Boost will cache the non-default-language.

This is something that almost every i18n site learns the hard way. I forget about it each time I set up a new i18n site and it bites me in the ass. Here is a patch that adds a requirements error with the above text. As a side-note this patch also improves boost's implementation of hook_requirements by allowing it to show multiple warnings/errors at once.