Trigger an error when a required include fails to be included...

I encountered this while attmepting to deploy a Roots-based theme to @WPEngine via git. A global `.gitignore` rule omitted the contents of `roots/lib/` from the index, so none of the `require_once` lines from `functions.php` worked. The `locate_template()` function can `require` the file, given appropriate flags, and returns an empty string if it can't find it. More useful would be an error message when this occurs. Whether this deserves `E_USER_ERROR`, `E_USER_WARNING` or `E_USER_NOTICE` is a subject for debate. Please advise.