[5.3] Allowing unversioned assets with elixir function

[assertchris]

Currently, when you have a file myapp/public/css/app.css, and use {{ elixir('css/app.css') }}, you will get an InvalidArgumentException, unless you also have mix.versioned('css/app.css').

This change allows elixir() to return paths to existing files, preferring to get them out of the manifest.

[assertchris]

I was watching this as you hit the merge button! My brain just wobbled in my head...

[piotr-cz]

You should also change InvalidException message to something like "File {$file} does not exist.", since now it's thrown not when $file is not present in asset manifest (it's not obligatory), but when it doesn't exist.