[5.x] Throw better exception when asset isn't found

[edalzell]

We are getting a ton of Call to a member function path() on null and it's coming from this code in Assets.php:

    public function process($data)
    {
        $max_files = (int) $this->config('max_files');

        $values = collect($data)->map(function ($id) {
            return Asset::find($id)->path();
        });

I'd like to know which asset is causing the issue, so let's throw a better exception.