Added exception if layout isn't found

I don't know if this first approach is valid, I want to discuss with you.

I have created a tree directories: 
- tmp/app/templates/posts/index.html.erb
- tmp/app/templates/default.html.erb


```ruby
require 'lotus/view'

class DefaultLayout
  include Lotus::Layout

  def page_title
    'Title:'
  end
end

module Posts
  class Index
    include Lotus::View
    layout :default
  end
end

Lotus::View.configure do
  root 'tmp/app/templates'
end

Lotus::View.load!
Posts::Index.render(format: :html)
```

With this implementation all works fine, if I rename my default.index.html layout to 1234default.index.html (for example). Now I have the MissingTemplateError error created.

Is this approach correct?
Don't you thing that `MissingLayoutTemplateError` reads better? ;)